It’s been openwrt, I’m not familiar with routerOs so it takes time for me to setup some things I already now how to do in linux. It has its own package manager that includes a lot of useful stuff. It’s just the way you see things: starting from configuring tools and using some programs (vim, mc …) and ending with nginx python and some other crazy stuff, that stock os doesn’t allow to do.
Try to do site to site VPN with your “standard vpn implementation” when 2 routers are behind (P)NAT and one of them (or none of them) can have port forward.