A single router might work, but i suspect might struggle to get a decent signal up into the loft.. you might want to consider a mesh system..
The two brands I would consider if i were in the market are probably ASUS and TP-Link.. Netgear could be considered alongside them, though I've never been a fan of their products..
Wifi-6 is only really relevant if you have wifi-6 capable clients. If most of your devices are only AC, then it won't make any difference whatsoever, but you may want to consider it for futureproofing.. whatever you buy should last a few years at least..
The biggest consideration with a mesh is whether you buy a dual-band or tri-band system.. tri-band is more expensive as it includes an additional radio that is dedicated to backhauling traffic between mesh points.. dual-band ones will share the radio that all of your devices use.. whether or not that impacts performance is really down to how many devices you plan to connect to the mesh.
Make sure whatever you buy has gig ethernet ports (rather than 10/100) else you'll limit the speed of the connection to 100mb.
Options in the TP-Link range are the M4 (entry-level) M5 (step up from the m4, but still dual band) or M9 (tri-band).. These are all AC devices. If you want to go for wifi-6 then consider the X20 or X60.
For ASUS, the options are somewhat more varied - many of their devices now support their "AI-Mesh" so you can buy a regular ASUS router (like the one you mentioned in your first post) and then add a separate access to the mesh at a later date.. or you could consider their specific mesh procuts - the premium wifi-6 product being the "ASUS ZenWiFi AX Whole-Home Tri-Band" or for regular AC the "AC3000 Tri-band Whole-Home Mesh WiFi System"
any of the options above are likely to serve you well.. 😉