@mysteriousname 

Those radiation graphs are pretty sophisticated, going by shape and/or colour doesn't necessarily tell the whole story.  I will admit in general the 650 looks better on paper, but from what I've read in this forum, sometimes performs less well in the field, particularly in the 'behind' zone.  I have been a long term user of the 225-outdoors which have a very similar radiation pattern to the 625 and have been happy with their performance.  You should know that the 650 uses printed antennas on the main PCB which is cheaper to produce, but not always better.  I'm pretty sure TPlink wouldn't have troubled themselves to 'late launch' an inferior product in the 6XX line, instead I think it fills some gaps namely  where mounting location presents some challenges and/or where the use of an external 3rd party antenna is required.  If you are just covering a suburban backyard go with the 650.  If you are trying to maximize your distance (especially forwards and behind) in difficult RF conditions, then my money would be on the 625 performing better.  You can see how the radiation pattern of the external antenna would also optimize a roughly linear 'line' of meshed APs as more of the RF energy is focused fore and aft with the externals than the 650 can manage.   The externals are also useful when mounting on metallic poles, especially wider ones like light standards.  YMMV, HTH.

Hi  @mysteriousname 

It's ~45° bent, because we recommend to use the antennas like that. 

In general, our antenna signals are radiated vertically and outward from the antenna. 

(Left) Scenarios that are not suitable for common level use

(Right) The router has better wireless coverage with horizontal use

A ~45° bent antenna can help you improve the coverage that "under"/nearby the EAP install location. 

  @mysteriousname 

You could modify the internals of the 650, but this will require some rather delicate solder skills to break out the signals.  The 625 gives you two standard SMA connectors from the get-go.  Of course, WiFi6 often uses 4x4 MIMO, to there may still be additional 2 antenna signals to break out regardless.  If you are already customizing, I'd start with the 625 platform since you don't care about the factory radiation patterns anyways....and the 802.11AX guts of both units will be similar if not the same.  You will also know how to better pair the 4 signals...often not obvious when staring at PCB antennas as they are the product of a whole lot of RF voodoo and this results in some non-intuitive patterns.

On the antenna orientation, a 90' angle between the anteanna dipoles allows for true horizontal/vertical spatial diversity which can improve the modulation efficiency between two points.  So in my opinion, it's less about coverage and more about radio performance.

  @mysteriousname 

I am not sure the stock radios will get you a 300-500m sphere of coverage (or toroid, or frisbee even). You get max 1W of power from the stock radios, if you do the math of 1W over the surface area of a 1km diameter sphere, it's not a lot.  You either need to amp up the power (and violate your local RF regulations), or you need to direct and focus the power using narrow beam antennas (ironically, these are usually PCB printed :) )

  @mysteriousname 

I am keen to follow along on this vicariously :)  Please share what you can as your project progresses.

If you are in a 'contained' space...tunnel, large warehouse, or even open water, or anything with some waveguide like properties (hard dense surfaces)...then your propagation characteristics will benefit measurably and make it easier on the poor radios!