Device for VLANs + mwan3 + AP


I currently have an "archer c6 v2" that performs really bad to load balance my wan (one 300Mb and one 50Mb backup provider), and a couple of 952Ui-5ac2nD to manage my IOT devices.
My plan is to keep the mikrotiks to isolate all the IOT devices and a good AP/router to do the load balance, routing and LAN/wifi. The mikrotiks are reliable, my configuration works well, but the max. throughput of this devices is low, enough for IOT but not to handle 300Mb connections. But the c6 doesn't handle the routing and wifi so well, I'm using snapshots and the performance is really bad, so I'm planning to replace it.
Now, I'm limited on the budget and models that I can afford. I live in Argentina, for example a WRT3200 is hard to find, only used, and cost ~USD350... too expensive... so I'm thinking on alternatives... maybe a c7 that has better support to provide decent 5Ghz wifi, and a raspberry pi4 to load balance...
I'm looking for some advises about what to buy, and what not, maybe some documentation about recommended setups... I really prefer openwrt over other solutions, I can find some mikrotik to do that job, or a combination of it, but to me openwrt is by far easiest and more maintainable.