Benefits of WiFi 6 with older devices

My house is a wood frame covered by drywall and stucco; typical for California. The wifi signal extends pretty far and I don't have any major problems reaching devices even outside the house. The problem I have seems to be related to having too many devices for the routers capacity.  I run between 20-30 at any one time which must be a lot of traffic for the "Wifi Highway" created by ny R9000.  What drew my attention to WiFi 6, in particular the RAX200 is the claim it can increase the highway width (maybe not so much its length). In fact the RAXE500 says it can handle up to 60 devices running simultaneously. What I am not clear about though is if that performance depends on using WiFi 6 clients (devices). Most of mine are WiFi 4 and 5 devices. Would I get that same level of bandwidth using Wifi 4 and 5 devices under the RAX200 or RAXE500? That's my main question.