converting from several grown 10Gbit links to a redundant topology

We (read: I; as a one-man-show in the company) are currently upgrading the network in our main branch by getting a few new/additional fiber runs installed between our buildings and to add a third building to the network. As we are not a tech-company bandwidth demands aren't that particularly high, although all services for our branches are hosted locally (main branch is multihomed), so redundancy is getting more and more important.

The current (10g) setup over 2 buildings looks as follows: https://imgur.com/a/EoHXvBa

The 10g uplinks have been added as bandwidth demands increased and are local to each building (-> each stack of 3750x represents one building).

As a third building will be added to the Network shortly, I'd like to clean up the 10g topology and probably add some redundancy; i.e. using 2 10g aggregation switches and redundant 10g links for the switch stacks and servers/routers roughly like this: https://imgur.com/a/KCfOHoE

As already mentioned, we're not a tech company, so my budget is rather restricted (Hence the stacks of used/refurbished 3750X). I'd really like to use cisco gear for the 10g aggregation, but getting approval even for some SX550X-12F, let alone some halfway recent catalysts with a decent amount of 10g SFP+ ports is just impossible. The current 10g links to the switches are through 3x c3kx-nm-10g modules. When adding the 3rd building I'd like to move one to the new 3750x switch for that building to cut some costs here, so 2x 10g uplinks are available per stack.

I've come across the mikrotik CRS3xx series switches, but I couldn't find any useful information about them supporting any form of L2 multipathing, so i suppose they don't support it?

I've never dealt with multipathing outside of routing or FC in my homelab, so I'm not even sure if this is even feasible or useful at this small scale. In cisco documentations multipathing is usually directed towards nexus platforms and/or strict 3-layer hierarchy involving only switches in the multipathing topology, not endpoints. Also the 3750X /w IOS 15.4 (ipservices license) don't seem to support L2MP.

I'd be very thankful to get some advice about a) If the redundant topology on the second diagram is feasible and/or b) If/how multipathing can be handled when using mikrotik CRS3xxs

Just for completeness: I don't think this topology can/should be handled purely with STP, correct?

Or would you advise against multipathing/redundancy and towards using a single 10g Switch? Any advices on what would be a good addition to the 3750Xs we're already running? (Reasonable power consumption is a criteria, so Nexus 5010/5020 are sadly out of discussion)