You can try to 'limit' the number of free users by doing an 'invitation to test your product', so you can test if your 'convert ratio from free to paid' is enough to sustain this kind of business model.
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Maybe this month you limit invitations to 1K, and see how many converts to paid users before the end of the month or you can just make a quick math if the standard is 3-5% of convert ratio then can I sustain this model with a 3%-5% and still make a profit?
If you have experience working w backend and servers then digital ocean is OK, you can get 3 months free w their free trial. But if you want to work without worries about devops then heroku is a great choice and is free with 1K hours per month OR you can go with AWS lambda (free tier) and enjoy a little bit of both.
Yeah, I just rush it. Haha