I see chain=input and chain=output in your mangle rules, but I use either chain=forward or chain=prerouting for marking routes for clients connected to the router.
And since you do not have a default route with no routing-mark…
Only port 80 is redirected for the proxy. The dns request (port 53) and the ping (protocol=icmp) must go through the router (chain=forward). Mark that route or set a default route without a routing mark.
FYI: This is not support. This is a user forum. We help you for karma (if not for ego, then basically free).
Please put all entries in one place. If you scatter them all over your post, like “/ip route” above (two places), then it is hard to see what you did. I see you are rotating the default routes if there is no routing mark, but that also means it could be the ISP for one of those connections. If it it slow about half the time, that is what I would check.