One router will be fine and the AX3 should have more than enough power to do that.
You will just create a separate VLAN for neighbours traffic. .
Setup firewall rules to disable traffic between networks. .
Disable Fastpath for traffic to/from internet and create queue’s for shaping. .
I would have done it with Simple queue’s - One parent default queue with max-limit 95M/19M buffer size 4096K. Your connection SFQ queue with limit-at 45M/9M and max-limit 90M/18M. Neighbour SFQ queue with limit-at 45M/9M and max-limit 90M/18M. With this config you and your neighbour can borrow speed from eachother when available. Just do some speedtest to ensure that you actually allways do get 100M/20M else your shaping will do nothing.