@MatteoRighetti Ethernet is ALWAYS the best. I have an industrial/commmertcial L2 & L3 managed switch in the attic that has 16x 5Gbps RJ45 connections & another 4x 10Gbps + 4x 10Gbps SFP+ connections. I go from both ISP to my switch, which being both L2 + & L3 includes the ability to solicit DHCP from ISP or be assigned a static IP. I have one of each and they are aggregated as a 3.5Gbps 802.3ad LACP connection for better bandwidth and redundancy/failover. This switch receives my public IP's and passes the connection to my lab and another switch, The commercial versions of these high speed (5Gbps - 100Gbps) are not inexpensive, however many offer consuumer versions that are less complicated and becoming less costly every day. This also gives you the ability to sniff packets at the source & control flow via ACL rules & by utilizing VLANs. 

In my home lab for instance, my primary switch feeds a pair of ethernet connections for LAN to a consumer QNAP L2+/L3 managed switch with 8x 2.5Gbps RJ45 connections & another 4x 10Gbps combo RJ45 & SFP+ ports. This switch is assigned a static private LAN IP & includes firewall-like ACL rule capabilities & you can also aggregate these links as well. Except this switch was $200, a quarter the cost  of my primary commmercial switch. This 2nd switch is feeding a dumbe 8-port 5Gbps switch along with 2x workstations, 3x physical/on-premises Linux servers, 3x Physical Windows Servers & a handful of VM workstations or VM/HV servers at any given time. I host my own authortative DNS on a pair of DC's with a backup via Hurrican Electric's free DNS, set up as a slave to 2x of my domain controller DNS servers. I also use a TP X6000 & 3x TP Deco X60 as AP's, prefering to let the more powerful equipment handle DHCP as well as DDNS for the one DHCP service. I am getting away from the point, so I digress....

I will say however that my setup has the added benefit of maintaining a high speed LAN network (rewired all with shielded and plenum rated CAT8 cabling) so if one or even both ISP's are down I still have high speed access to all my domains, my DLNA & streaming servers, backup functions, SMB over QUIC & my ISCSI disk drives. Plus the entire LAN & VLAN's no longer care about the ISP side. Even a new public address and even if using IPv6 tunneling, it doesn't matter, because everything past switch one is static IP when it comes to IPv6. 

As much as possible use ethernet to connect switches, AP's & mesh network components. You get much faster and more reliable connection using ethernet backhaul (NOW THAT IT'S WORKING AGAIN, YEAH YOU, TP-L!). Otherwise, you can probably move the second Deco farther from the first, but it's hard to tell, since your drawing does not include a scale for distance. Also, not meant to nitpick, purely a suggestion, whenever you take the time to diagram something, it is very helpful to at least use widely recognized symbols for the equipment in question, if not everything. There are cheat sheets online that engineers and designers use daily & it can be a huge time saver & serve to convey both layout and provide a PID like understanding of how you've arranged any equipment in the system. Sometimes placement and ordering matter immensly. Probably not here, but still took a moment of zooming around to realize your blue highlights were probably windows and doors & not very pertinnent. 

As for Powerline adapters, it depends on where you are and what construction standards were followed during the construction or remodel. If the unit are on the same circuit, they can be a suitable solution. However, depending upoon the electrical design and equipment in place & how they structure is wired, they may not function very well, if at all. In my experience, they do not work very well if the circuits are not adjacent & that can be further complicated if neutral wiring is present & if it was wired correctly as individula circuits or if they were lazy and in area of multiple fixture they simply connected all of the "neutral" lines at a single switch/receptacle or J-Box. The later can help with some of the issues regarding Powerline accessories, but can also wreak havok on some light/fan switches & particularly smart switches and dimmers, 

Lastly, have you tried dedicating 2.4Ghz to the inter-mesh device network & use only 5Ghz for device connections in the Gym? 2.4Ghz has far better (Evidently a word meaning "HOLE THRU AN OBJECT is banned from use,; COME ON TP-L, SMH) so forgive misspelling penetrat!on thru walls and such.

GD IT, Serriously???? I cannot say "first letter oof alphabet" + hole to indicate a solitary opening thru a wall or other structure??? Jeez, You guys sure love to make it hard to assist anyone with anything, I'd frgotten that & the reason I usually don't bother...