Routing advice for 5G PtP link

I've got a bit of an odd issue, looking for suggestions, thank you!

I recently joined a 150 employee company with two buildings, 500ft apart. Second building was recently acquired, currently 120 employees in B1 and 30 in B2.

Each building has a 20Mbps (i.e. slow) ISP connection. Currently there is an ipsec tunnel between Fortigates at each building. B1 has on-prem VMs that B2 users connect with. Each building is a separate subnet and communication is routed via the Fortigate ipsec tunnel.

Main problem is that there are some users in B2 that need to access/transfer large amounts of data (SQL, MP4s, etc) on VMs in B1. We have a 400MB test file we use as a baseline which takes over 6 minutes to transfer over the ipsec tunnel. (Same file on same subnet in B1 takes only seconds of course.)

Upgrading the ISP lines is not an option. Moving the impacted users to B1 is not an option.

I set up a test 5G PtP between the buildings, which is working great at around 700Mbps. Our 400MB test file takes 15 seconds.

So, I'm wondering what the best solution is to make this work?...

Only a few users (5 ppl) in B2 need this solution. What is the best way to give them connectivity to BOTH the existing 20Mbps ISP connection (subnet A) and the 5G PtP wifi connection (subnet B)?

I'm thinking, connect the 5G to the B2 Fortigate, then create static routes for the appropriate servers and route that traffic through the 5G?

Any other/better ideas? Thank you in advance.