I always get confused with AGPL and seems to have differing opinions online, so wanted to ask here. If I use Lago in my app, and don't change it all, do I need to opensource my entire application?
That is - offer your code to your customers under the agpl, not necessarily back to Lago or the community. But, since your customers get the code under AGPL, they in turn are free to distribute it. So, in practice, going that way - it generally makes sense to just open it up, and contribute back directly.
(imagine your only customer is IBM, or some other big company - they might not care/have an interest in re-distribution of the AGPL code - so you could be compliant (offer source to IBM) - but effectively closed off from the community).
I think what matters more is how the creators of the software intend the license. In case of Lago it's clear they want you to use it this way, and AGPL is only really meant to restrict businesses that want to fork it and start a proprietary billing service.