That sounds reasonable. I hope it works out. Personally, I’d love to make a living running an open source project and I wish you every success. Hashtag live the dream 
Even though I don’t desperately desire the features you describe, I’d consider paying for Vikunja. It depends on the cost.
One thing that drives me bonkers about software that has both paid and free tiers is when they don’t make it clear from the link that it’s a paid feature. I click on it all excited, then it goes, “Actually you need to pay.” I feel like they’re trying to trick me. Fitbit is guilty of this. But if a menu item were something like, “Audit logs (paid)” that wouldn’t bother me. As a user, I understand that you have bills to pay and you’ve already given us a great deal for free.
Best of luck!