Part of the Furniture
What a crock of stuff. Even you have stated the router is smart enough to choose its own best WiFi settings. And shorter simple SSID's are better if some clients have issues. Dual Band SmartConnect also works in these modern routers.@TCoreX, hopefully you didn't reset your devices yet. Don't touch the devices.
If you played a lot with your settings - reset the router only and start fresh. Set your usual SSIDs and use your usual password. If a device doesn't connect - re-connect it manually with WPS, Forget Network or whatever is there. This is all you need to do. Make sure your password doesn't have spaces - some Asus routers don't allow that for weird reason. A space is a valid character, but... someone decided otherwise.
Set 2.4GHz this way and test:
[Wireless Router] How to improve compatibility of IoT device with ASUS WiFi 6(AX) Router? | Official Support | ASUS GlobalUse 20MHz wide channel, set the router on Auto and observe the logs for few days. Lock the channel the router uses more often on Auto - it usually has the higher bandwidth available. Use any channel 1-13 and not 1-6-11 only. Disable all Beamforming, MU-MIMO, Airtime Fairness, non-standard Turbo/NitroQAM Broadcom only specific.
Set 5GHz this way and test:
Channel 100, 80MHz wide, uncheck 160MHz, WPA2, disable all Agile Multiband, PMF, TWT, Airtime Fairness. Channels 36-48 in EU are limited to 200mW transmit power. Most of the guys above have up to 1000mW on 36-48 plus 149-161 available - you don't. Your router is the same model, but with different regional settings, hardcoded, can't change.
Disbalanced up/down rates - check the link speeds in logs. AC best is 866, AX is 1200 at 80MHz wide channel. This translates into about 500 and 800Mbps throughput, close to the router only, no obstructions. Houses in Europe are built differently - one wall may cut off you signal strength with 70%. You also have more reflections and your AP works harder. Faster download than upload happens when the device hears the AP clearly (let's say 200mW radio), but the AP can't hear the device well (usually about 20mW radio). Link rates and signal strength will give you clues. You need -60dBm or better (lower).
Don't test with mobile devices. They all have power saving features and your speed measurements may be inaccurate.
Don't use Wi-Fi Analizer apps not showing bandwidth available. They show only the channels the other radios around are tuned to, but not how active they are. Your best channel may be the one with other APs on it already. Also, the app runs on a client device and the environment is seen from the device's radio - not what your router sees around. The difference may be significant.
MU-MIMO and OFDMA basically do nothing for you. MU-MIMO may cut down the communication to compatible client to single stream. With your 40 devices I'm sure less than 10 support Beamforming, required for MU-MIMO. Enabled or disabled - doesn't matter much.
I'm using 4x APs setup in my 600m2 house in North America and also 4x APs in my 120m2 house in Europe. This much is the difference. The NA APs work on full power and cover big relatively Wi-Fi transparent area. The EU APs work on low power and cover room and a half in almost Wi-Fi blocking environment. You can't take advice from people in different environment and with differently set as per local regulations routers. This is a waste of time and you can reboot and reset forever with no positive effect.
The information provided from you is close to zero. In this case I can only give you suggestions, but you have to understand how whings work first. Otherwise you only follow wild guesses not even related to your setup and only waste your time. Take advice from people in Europe - @ColinTaylor, @BreakingDad, @TheLyppardMan, etc. Due to geopolitical situation you may live in increased radar activity area without knowing and it will also affect your Wi-Fi performance. What you know worked before may not work anymore.
Don't use SmartConnect for better device management. Don't use QoS - you don't need it on 500/500 ISP. Don't reset your router multiple times for no reason - not needed. This forum is 1/3 about Asus router resets and 1/3 about speed tests. Don't copy someone else's settings because it works somewhere else for someone else. Experiment and find your own best settings.
If nothing works - send the sucker back and get local professional advice about the right equipment for your needs.
You may be younger than me in years but an old person in practice.