My advice, for what it's worth, and based on my experience building and running https://openexchangerates.org, would be to continue to offer it free (in your case, open sourced) but provide the hosted version as a service.
For example: I'd very gladly pay a small monthly fee to use your API for my invoicing system which I'm working on right now. I already have a load of open source tech I rely on, and some things just make more sense to pay for (e.g. using GitHub instead of self-hosting something like GitLab - it's a tiny monthly fee that saves me hours of hassle). I'd much rather use a hosted API that I can integrate in minutes than spend hours potentially faffing about with installing stuff from the repository, and worrying about keeping it up to date, explaining it to outsourcers/team members, etc.
If I do end up using your solution in my biz, I'll gladly donate - make sure you have donate buttons up and prominently displayed!
For example: I'd very gladly pay a small monthly fee to use your API for my invoicing system which I'm working on right now. I already have a load of open source tech I rely on, and some things just make more sense to pay for (e.g. using GitHub instead of self-hosting something like GitLab - it's a tiny monthly fee that saves me hours of hassle). I'd much rather use a hosted API that I can integrate in minutes than spend hours potentially faffing about with installing stuff from the repository, and worrying about keeping it up to date, explaining it to outsourcers/team members, etc.
If I do end up using your solution in my biz, I'll gladly donate - make sure you have donate buttons up and prominently displayed!