Sorry, bit late to the conversation, but thought I'd throw my 2p in....
For background my house sounds similar to OP, massively thick walls mostly made of engineering brick (old 50s council build). Tried tons of solutions as getting wired throughout the house would be difficult.
A few years ago I ran a couple of XT8's, using wireless backhaul. When configured correctly these were excellent.
Then moved to Unifi. Tried UDM (awful), and various combinations of APs. Eventually we had an extension built and I build a full rack in the new office (part of extension); UDM-SE, couple of PoE 24 switches, UNVR with their cameras all round the house, managed to get wiring put in when the extension was built so put the little USW Lite's in a couple of rooms over PoE, and 2 APs. Tried combinations of U6-LRs, U6-Lites, and U6-Pros... one in attic and one in office (far lower corner of house). There was *always* dead spots that got poor to no wifi signal. On top of Unifi "updates" which cause problems all too often, I eventually decided to replace everything.
Ended up with OPNsense as my main fw/router, then a couple of TP-Link PoE switches (2.5GbE and 1Gb), and replaced the attic/office APs with EAP670s. *Massive* difference - no blackspots anywhere in the house, and speeds (SnR) significantly improved on all devices, including round the garden. Highly recommend them APs, so much better than any of the Unifi U6s I've had.
I also replaced all the Unifi protect cameras/system with some Reolink ones. They all get controlled/recorded by a mini-pc with a Google Coral device running Frigate (and recording 24/7 to a 16TB share on a NAS, over 2x2.5GbE links).
Had YouFibre a couple of months ago and they've given me an Asus ET12 - still in it's box but I meant to get it out and have a little lab play with it
TLDR; Found the Asus (using wireless backhaul) very good, and the TP-Link EAP670 fantastic - way better than all the Unifi APs.
Edit: Just to add my recommendation of the EAP670, they're so good that if I update (or switch off) the AP in the office, I still get a signal from the AP in the attic. That is about 25-30m away, two floors up, and going through at least 4 thick walls. It's a weak signal, but I stay connected. Tested the exact same with a U6-LR and lost signal way before I got into the office.