What is the best way to implement a scheme with two providers?

Question

Computer networks

Greetings to everyone, tell me there is the following scheme, I will try to describe it in more detail:
There is a single local network spaced apart in 3 different buildings (highlighted in green), somewhere launched between each other via optics, somewhere via copper, and 2 providers are connected at different ends of this local networks. The task is the following:
1) It is desirable to run that the client/s physically plugged into the CRS-125 use ISP1.
2) Users plugged in D-Link 808HV and RB-951 used ISP2.
it happens that communication is rare, but it can be lost between Mikrotik CRS-125 and RB-951, so they need to work independently on different ISPs. But!
3) Most importantly, you also need to consider that if all parts of the local network are working, but one of the ISPs falls, everyone automatically switches to the second ISP that is available.
How to implement this scheme more correctly (I hope I could describe it more or less normally)?
It seems to me, and as I depicted it in the diagram, organize a VLAN between two Mikrotiks so that ISP1 is good to RB951, in which 2 ISPs will already meet, and on it already make a Failover version. But now I don’t really like it because if the connection between two Mikrotiks is lost, then the client on the CRS-125 will be left without the Internet at all.
How to do everything more intelligently? Tell...
Maybe this is somehow solved by dynamic routing or, for example, the DHCP server will monitor and replace the gateway if part of the network falls off, or something else?

Answer 1

Good day.
1) Physically, I would not, better at customer addresses:

/ip firewall mangle
add action=mark-routing chain=prerouting disabled=no dst-address-list new-routing-mark=via_crs125 passthrough=no src-address='адреса клиентов которых выпускаем через crs125'
/ip route
add disabled=no distance=1 gateway='шлюз ISP1' routing-mark=via_crs125

2) The same only via_rb951, ISP2 gateway and addresses of other clients
3) Create one rule on both microticks for their ISP

/ip firewall mangle
add action=mark-routing chain=prerouting disabled=yes dst-address-list new-routing-mark=via_crs125 passthrough=no src-address='все подсеть'
add action=mark-routing chain=prerouting disabled=yes dst-address-list new-routing-mark=via_rb951 passthrough=no src-address='все подсеть'

And two rules in /tool ​​netwatch:
- ISP1 has fallen, down-script - disable the rules from points 1) and 2), raise the necessary rule from point 3); up-script - vice versa
- ISP2 has fallen, down-script - disable the rules from points 1) and 2), raise the desired rule from point 3); up-script is the opposite.