Redesigning a school network

Hello :)
I am currently trying to redesign a network in my old school in my hometown. It's all a big mess, I have to identify each cable where it goes and mark it.

And the thing is they have 4 ISPs, one of them connects all the classrooms and another one connects the administrative computers, while the other two are completely unused.

I was thinking to leave the classrooms connected to one ISP, and join the other three through a OPNSense, with load balancing and failover. I have a computer good for installing OPNSense, the problem is it has only 2 PCI-E, and I need some network cards with more than just une RJ45 port. Also, it seems they are pretty expensive and I don't know how they are compatible with personal computers.

Other option is buying a router like a https://mikrotik.com/product/RB2011UiAS-RM which is a little bit more complicated to configure. But I don't know if it can handle the job, considering the connections are 1 Gbit down/up, 500 Mbit down/ 25 Mbit up, and 100 Mbit down/up, and maybe 10-15 computers using it at peak times.

I would like to find other opinions, how would you see this done and if you think the Mikrotik would be a good option.

Thanks a lot!