Use the signal source and connection preference to adjust for the devices part

yes, use the deco app to add in deco x10 or E4

Yes please

Perhaps that is the maximum it can connect to reach contrary to the website details about it reaching up to 100 devices. Consider upgrading the device then

have you tried turning off all 6Ghz, 5Ghz, just only turn on 2.4Ghz? Beamforming, fastroaming or whatever wifi features, just turn it off. Try it specifically once again with just 2.4Ghz, and...

Login in the deco with browser, there's system logs, though I'm not fit to read only the TP-link engineers can decipher the logs.