Always looking for some good management software to handle lots of systems. Open Source NMS integration is definitely a plus 
Some useful search terms for finding software like this: isp billing software open source mikrotik wisp
Trying to put together a resource in a google sheet. Please add if you have something useful:
https://docs.google.com/spreadsheets/d/1aSjm0M8LA0cllSg4IkIiBCT5NFmN_NKj3HznBQUrRyE/edit?usp=sharing
Thx
David