ac (wifi 5) is old.
ax (wifi 6) is the current norm, with wifi 6e available (although expensive) and soon wifi 7.
other devices slow down wired connections too.
legacy devices can go on 2.4ghz (or a second or even third 5ghz band if available). mimo also helps.
it's slightly less. the difference is not significant in this context.
again, ac is old.
real world wifi 6 (ax) speeds at 10 feet, topping out over 1.6 gbit/s:
real world wifi 6 (ax) speeds at 40 feet, topping out over 1.4 gbit/s (although a different router):
note the sharp drop-off for routers with gigabit ports, which top out just over 0.9 gbit/s, a little more than half that of the fastest multi-gig router.
there isn't that much interference, most clients support 80 mhz and two mesh units can use a 160 mhz backhaul.
and i'm going to stick to mine, that that it's not that clear cut.