I can recommend the TP-Link AX73 for good coverage from personal experience. ...
In my experience, in urban interference conditions, all 2.4 MHz devices work about the same, especially considering that on the opposite side the same cheap decrepit device responds to the expensive router, this also applies to Bluetooth. However, there are outstanding examples that work very poorly. I had one, called Huawei. It was on Realtek, with 2 antennas, but I have never seen anything worse. If you have such crap, then replacing it with anything will give a noticeable result.
... I can use an older router to handle the PPPOE and LAN connections then have a separate router just for the Wi-Fi stuff.
MediaTek has a 100 Mbit network, so any old junk will do. I don't even remember its IP address, recently I wanted to enable the 5 GHz mode, which is disabled. Both sockets work in parallel, dhcp is disabled, everything unnecessary is disabled. Only AP mode, and this mode works flawlessly.
... If spending a bit more money on better gear is not an option, the best you can do is do a site survey and plan your channels/channel width carefully. ...
Try to make your high jump 10 times higher for money.
Use the Ubiquiti WiFi Man application on your phone to help you select better channels.....
On 5 GHz I don't see any problems with interference at all, but this range physically works not far. I can go to my neighbor with a dect phone, but the 5 GHz connection breaks off in front of her door. And on the 2.4 GHz range there is a choice between 3 zones. To make a choice from 3 possible - of course, you need a huge computing power, you can't do without a program. I recommend always setting channel 6 manually - I always do this and have never made a mistake (2.4 GHz).