@ConcernedUser 

As a happy user with the Deco set of M9 Plus and M5, who tries but can't find justification to spend money to upgrade for newer models, here are my thoughts.

250Mbps Internet speed can be handled just fine by WiFi5 Deco, which is M5. The speed of Satellite Deco will unlikely to increase in a meanigful way if you buy newer Deco models. No will coverage: faster protocols such as WiFi6 mean shorter distance.

Powerline Deco such as P9 and PX50 are often hit and miss, as they depend on electrical wiring with its noise. If you are talking latency and stability, don't go with those.

What could help to increase speeds on Satellite Deco is tri-band capability, which M9 Plus has (but M5 does not). In my tests with both M9 Plus and M5, I am getting at least 30% faster speeds on Satellite M9 Plus than on Satellite M5.

Tri-band speed improvement works only when both Main and Satellite Deco support it. If 30% speed improvement sufficient for you to justify buying Deco set, I suggest you buy 3-unit set of M9 Plus (if you can still get it) and deploy in place of three Deco units you have.

Then, put aside S4. Use two M5 you have to expand coverage to places where you need WiFi signal but can sacrifice speed. These M5s can be connected, using Connection Preference feature (https://community.tp-link.com/en/home/forum/topic/624082) to Satellite Deco if they can establish better WiFi link with Satellites than with Main Deco. For example, if you need to add WiFi coverage to some place upstairs, you'll put M5 there and make it connect to Satellite M9 Plus upstairs.

If 30% speed improvement can't justify spending for you, just put upgrade plans on hold till you get faster Internet link.

  @Alexandre. Thanks very much for the response!
Honestly my main concern isn't speed. Like you say, the current Deco's are handling speed fine.  As for Powerline Deco, I thought they combined powerline with wireless to create a stronger connection between Decos? Would I actually get more latency? I'll avoid if that's the case. 

My main thing I want to improve is latency. I don't particularly like that my connection between Decos is "Medium". I feel it should be "Strong", as they're not too far apart (imo) and also because I'm unable to move them anywhere else really.  I think getting a Deco that could shift this connection to strong would be worth it, hopefully reduce latency too and give better coverage, but with what you say, I wonder if any of these models can offer that? It was certainly my hope. 

Then, put aside S4. Use two M5 you have to expand coverage to places where you need WiFi signal but can sacrifice speed. 

If I get new Deco's, I'd be replacing the M5's. It's a little sort of upgrade scheme where they'd have to be traded in so I'd no longer have them.  Some models are cheaper to upgrade to than others, the X10 and X20 aren't too expensive for example, the M9 Plus sorta is. 

Given you likely went from the M5 to the M9 Plus, can I ask what was your experience going from Homecare to Homeshield? That's one of my worries too.

  @ConcernedUser 

I run Deco mesh with both M5 and M9 Plus. I tested them both. I can tell that M9 Plus is natural upgrade for M5.

My Internet speed while I did own Deco went from 300Mbps to 500Mbps to gigabit.

At 300Mbps, my Satellite M9 Plus provided full speed of Internet connection.

At 500Mbps Internet link, Satellite M9 Plus started to reach its peak performance, I was getting between 350Mbps-450Mbps from it.

With gigabit Internet, Main Deco started to show its limits. The maximum I ever got from it over WiFi was 650Mbps.

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If you have 250Mbps Internet speed, with properly placed Satellites M9 Plus they should deliver this speed.

Signal Strength on my Satellite M9 Plus is always Medium. It is upstairs right above Main Deco, with only wooden floor/ceiling between them. Medium Signal Strength by itself not necessarily a sign of issues.

I don't use Homecare or Homeshield. My ISP provides these types of services for free (comes with the price of Internet package).

My plan was to update my Deco mesh with X50-POE. They support WiFI6 and have similar to M5/M9 form factor that I like. X50-POE are not tri-band, which will end with less speed on Satellites comparing to tri-band. Still, because WiFi6 can provide gigabit speeds and above, I would expect Satellite X50-POE delivering at least M9 Plus speeds or better.

The reason I am holding with upgrade is because my set of M9 Plus and M5s works fine. I don't have complains from family members about Internet speed and quality. What WiFi speeds M9 Plus and M5 deliver in my household are more than enough for everyone.

  @Alexandre. Thanks again for the response, appreciate the info. 

Alexandre. wrote

If you have 250Mbps Internet speed, with properly placed Satellites M9 Plus they should deliver this speed.

Signal Strength on my Satellite M9 Plus is always Medium. It is upstairs right above Main Deco, with only wooden floor/ceiling between them. Medium Signal Strength by itself not necessarily a sign of issues.

In terms of speed, my M5's are pretty much solid, so it wouldn't be worth upgrading purely for that for me.

Signal strength doesn't pose a huge issue for speed, but where I do have a problem is latency, particularly for online games but also in other areas. 

It's something I would imagine having a stronger connection between Decos would help fix, right? 

The thing about the M9 Plus, is that on the upgrade path, they're one of the more expensive options, and it's a price that I'm not sure is worth it. It's not good to hear that 1 floor apart you also only get a medium connection with them.

For the X50, X20 and P9, the prices are quite reasonable, a bit over half the cost in comparison. 

My network is mostly fine but I do want to reduce latency and slightly increase coverage if possible, as there are some spots where it's not ideal. I'm just not sure what models to go for to achieve this. 

I don't use Homecare or Homeshield. My ISP provides these types of services for free (comes with the price of Internet package).

 

Lucky! Mine is pretty much useless. The shift from Homecare to Homeshield gives me a lot of pause in upgrading to a new Deco model. I don't want to lose features I'm currently getting for free, but I wonder if it can become tolerable. 

I've been doing some further research, and found that the Deco X60 has 4 streams, which I think means it's more capable of maintaining full speeds across wired mesh nodes, while the X10/X20/X50 only have 2 streams.

Does anyone know how many streams the M5 has in comparison? I seem to get full speeds per node. 

I also know it has 4x internal antennas, in comparison to the 2x in the X10/X20/X50, but I'm not sure if that has real world relevance or is linked to this in some way. 

  @ConcernedUser 

The M9 does not support IOT network, Device isolation and does not recognise Tapo devices.

I returnera three M9̀s after discovering this.

  @Ache Thanks very much. I'm largely put off from the M9's at this point. From what I understand the X models generally perform better. 

What Deco did you end up purchasing instead of the M9? I still can't decide between which models to go for when my main goal is reducing latency and I can't use ethernet backhaul

ConcernedUser wrote

  @Ache Thanks very much. I'm largely put off from the M9's at this point. From what I understand the X models generally perform better. 

 

What Deco did you end up purchasing instead of the M9? I still can't decide between which models to go for when my main goal is reducing latency and I can't use ethernet backhaul

  @ConcernedUser 

I decided to wait as I didn't find a Deco meeting my requirements.

Technically the X50 family can be a possibility but then I started searching for posts and decided to wait.

I have asked @TP_Link for advice but received no reply.

Best regards

  @ConcernedUser More Ethernet cables=better network performance, so if you can at least connect the nodes via Ethernet that will work much better.

  @McFly That would be the ideal scenario but unfortunately it's not really an option for me currently.