The easiest way out would probably to relegate the xr500 to mere AP duties, while offloading the routing and sqm/cake functionality to a small 4-port (wired-only) N95/ N100 x86_64 mini-PC (price range around 130-250 EUR). sqm/cake is a heavy burden for your router and ~190 MBit/s is within expectations for that (NSS only comes with an offloaded fq_codel implementation).
If you are fine with the performance of the NSS based community builds -aside from the missing MAP-T support- you should be able to build OpenWrt from source, using the community builds as a basis and enabling the additional packages you need for MAP-T to work. Building OpenWrt that way should be less than half an hour's worth of effort, to be repeated whenever you want/ need an update, not great, but an option.
…but the easiest option remains to use routing hardware with enough raw performance to do its tasks without batting an eye (x86_64, RPi4, rockchip, etc. - yes, all of those are effectively wired-only, but they have the performance and you already have a great wifi5 AP in your xr500).