the rourter is very cabable of doing what you are looking to do.
you have enabled SQM of course? it is the most simple way to address compettion for bandwidth.
certain wifi tweaks help some people too. as mentionsed, htere are many threads about this; here is one:
https://forum.openwrt.org/t/linksys-wrt3200acm-vs-netgear-r7800/49944