Fitlet (x86_64) + unifi 6 lite testing / tuning?

Hello community,
I've recently began putting together a replacement setup for my 76sq / meter apartment in a mostly timber house.

Currently I'm on a 1Gbps up/down fiber connection.

The new setup I've finally finished putting together is a fitlet-X-LAN Barebone with OpenWrt 21.02.1, r16325-88151b8303 and after a bit of struggle installed OpenWRT on the Unifi 6 lite as well and configured it for Dumb access mode (I think, I'm not entirely sure if it's that or bridged AP mode).

There is also a HPE OfficeConnect 1920S 8G connecting it all up and providing POE to the AP

Everything seems to be functioning well, I've been running netperfrunner scripts about 2x daily for the last few weeks to measure throughput and bandwidth from the new router itself, before there's any load on the network to get an accurate idea of it's capabilities, I regularly get around 900 Mbps up/down.

This entire setup will be eventually replacing my ageing but very loyal Netgear WNDR3800 which has served well since roughly 2011, but I fear is probably choking now on too many simultaneous network connections, plus I look forward to playing with Wireguard and other fun features running a true x86 box will bring.

My home network consists of a decent QNAP box hosting some external docker services (minecraft, starbound multiplayer servers and a Jellyfin media server), one gaming / main workstation and a few laptops cellphones.

What would be the best method for comparing and tuning this new setup before switching it in fully.
I'm also specifically looking for tuning help regarding SQM and if I should be pinning processes to one core as I've read elsewhere that can affect performance greatly.

Lastly another strange phenomenon I've been noticing is that while testing bandwidth from the router itself tests close to spec (900Mbps up / down), doing any speedtest from devices connected directly to the setup get much much worse results depending on test: 90 down 150 Megabit/s up 350 down 200 Mbps up 500 down 350 Mbps up (this seems to most be testing to the fiber hotel in country)

Any idea what gives? is it normal for speeds results to range as various as this?

Any advice appreciated.