Layer 2 link redundancy?

Hey all, apologies in advance, I'll try to explain this the best I can.

First of all, I'm using Ruckus (brocade) switches, mainly 7450 and 7250 on SPR code.

I have two properties connected by layer 2 Fiber. It's two links in a port channel (link aggregate group) operating at 20gig.

We want to introduce a backup layer 2 connection. However, it's a much slower (300M) point to point wireless connection.

I know that I can't just add the point to point to the LAG because the speed of the whole lag will be limited to the speed of the point to point.

I've been researching using spanning tree for redundancy, but I'm not really sure how that works.

Basically I want it to work like this: 1) Use the fiber lag always if the link is up. 2) If one fiber link goes down, the LAG will still be up, we'll just operate at 10gig instead of 20gig. 3) If both fiber links (the whole LAG) is down, use the slower point to point

Any recommendations? Being on layer 2 for this link is what's getting me all hung up.