Looks like the WiFi mesh was the right solution for the horrendous WiFi coverage in the new flat.

3 little mesh routers later, and I have excellent speeds everywhere on both floors, and, more importantly in the office where I edit / publish stuff.

@thelinuxEXP I set up a two router mesh last month and the roaming works ootb even on linux, but I've noticed that my desktop has ping spikes sometimes up to 200ms, which is fine for using youtube/downloads but not for gaming, so;
even though it's a bit tedious and im not sure if there's a way to automate it, i switch to my slow but consistent latency powerline (even though its min latency is actually about 10ms higher) when gaming and fast wifi when downloading stuff but don't consistency.