I think the problem is in:
/ip firewall mangle add action=mark-routing chain=prerouting dst-address-list=org-a-intra new-routing-mark=org-a-intra passthrough=yes
You have to put src-address-list=org-a-intra instead of dst-address-list. Try it

I think that you have to route all the traffic from the pppoe server to the mikrotik that do nat without going to /ip firewall so without doing scrnat whit the pppoe server. In the mikrotik nat put in /ip firewall the ip's of your pppoe client and done you have your setup. I have the same setup, her...

Hi
You have to use mark routing with the l2tp client ip's and mangle rule another mark routing with the pptp client ip's so you can go to ip/route and add l2tp mangle in the route1 and pptp mangle in the route 2 or what ever you want to do

Check if the concentrator allow your mac address because it can block all the mac address except the first router. If it this you have to change the mikrotik mac with the first router mac

I think that the VPN mangle is wrong, it isn t dst address list VPN but scr address list. Try to set scr-address list to the VPN list.

Maybe you need a dns filtering tool like pihole or try something with cisco umbrella dns filtering