There could be a number of reasons things slow down.

You’re off to a good start. I would get a second router ONLY for the smart/automated devices. Someone else mentioned ethernet and that you didn’t want to. But a good rule to consider is that you only use wifi when you have to. If it’s not too much to run ethernet do so. Get a switch to give you more ports.

The errors you see are telling you its their stuff. Don’t buy a new thing. You just spoke to a dummy.