28k€/month is not enough revenue to keep all the people who are working on Mastodon. Donations can only work if we assume that there will always be a constant flux of people willing to work for free, dealing with all the unpleasant things that most FOSS developers rather not do.
The moment you factor in the costs of employment benefits (to cover their vacation time, sick days off, fund their retirement, health insurance...) and taxes, the 4k€/ brutto quickly becomes 2k€ net.
I just hope you understand you won't be the one determining what is "more than enough" - the market is, and the market is paying a lot more than 25k€/year for any decent Javascript/Rust developer. If you have people that live in areas with low cost of living and are okay with being severely underpaid for some higher purpose, then maybe you can pull it off. But it's going to be basically impossible to find good people willing to stay for the long run with that attitude.