MikroTik

I have a a Mikrotik router on building A with WAN access and 3 LANs.
It also has a link to building B where I have just installed a new Mikrotik router.
On this building I want that some of the users have access to one of the LANs of building A. And the others I want to have access to Internet but with different bandwidths.
So, my idea is to create PPPoE accesses to each user of building B, with queue simple-ethernet to manage the bandwidth and IP firewall filters to drop any communications between the several networks.
Is it the best approach or is there any better solution?
Thank you

If you're comfortable with RouterOS, this will not be too difficult. I recommend you make a diagram and it will really take shape for you and help us too. You mention PPPoE. Are these Buildings separated over VPN WAN or private fiber?

I have to guess, because you did not state, but every building has a router and thus each building is on a different subnet. That helps, so simple rules in Building A's router could mangle traffic that put marks on it, which you can then use to put them into queues and prevent access to LAN 2 at Building A.

To really organize things, you need the three LANs at Building A to be either: on a different IP scheme, different port on the router, or in a different VLAN, so you can create rules to work with them. This is true of every network you have now, or will have in the future.

If every business group was connected to a different port on a router, you could segment that way. If everything had a different IP scheme, or was in a different VLAN, you get the idea ... which is correct? Well, that's kinda a personal thing and you probably have a hybrid.