Some info may still be missing (e.g. it’s not clear what the VPN is about), but it’s better. Well, in a sense that it’s possible to see what you have, but otherwise it looks completely wrong.
Route #3 looks useless, but it doesn’t break anything.
Address 192.168.100.10 on bridge and everything with it doesn’t make any sense. If you want to forward ports to remote 100.80.20.75, why do you forward them to local 192.168.100.10? That’s dstnat, and I don’t even know what to say about srcnat, because there’s no way how those rules could ever do anything useful.
If you want to forward ports from 103.x.x.x to 100.80.20.75, you want to do:
/ip firewall nat
add chain=dstnat in-interface="ISP A" protocol=tcp dst-port=22,80,443 action=dst-nat to-addresses=100.80.20.75
(I don’t like in-interface=“ISP A” and I’d prefer dst-address=103.x.x.x, but since it may not be static, this is ok too)
That’s basic config for router A. And now you have two options:
a) If you’re interested in keeping original source addresses, then this is all for router A and rest needs to be done on router B. You basically need dual-WAN config, where second WAN is the link to router A. It means another routing table where router A is default gateway, mark new incoming connections from router A, and then mark routing for responses to go back the same way. Something like:
/ip route
add dst-address=0.0.0.0/0 gateway=192.168.10.2 table=to-router-a
/ip firewall mangle
add chain=prerouting connection-state=new in-interface=<interface connected to router A> action=mark-connection new-connection-mark=from-router-a
add chain prerouting in-interface=<interface connected to servers> connection-mark=from-router-a action=mark-routing new-routing-mark=to-router-a
b) If you don’t care about source addresses, then you don’t need to do anything on router B and you can simply do this on router A:
/ip firewall nat
add chain=srcnat out-interface=ether3 src-address=!192.168.97.0/24 action=src-nat to-addresses=192.168.10.2
Some other tweaks may be required, but it depends on what other config you have.
