Once you take money, you are running a business (even if you mislabel the money as a donation). That means having to register with the tax authorities, adding a ton of accounting related overhead and very likely spending money on all of that overhead (e.g. when hiring an accountant to offload some of the overhead).
At some point you realize that you have to ramp your business up, just so it pays for the cost of accepting money. Then it's no longer a side project.
We actually have something like that in Germany for musicians: GEMA.
Unsurprisingly, it is a failed experiment. Among other things, every time you buy storage media that could be used to store music (burnable CD, DVD, ...), you are forced to pay a GEMA fee because you could use the medium for piracy (which, of course, remains illegal nevertheless). Everyone publicly playing music has to pay GEMA fees, because he might play something from the GEMA portfolio. As a musician you more or less need to join their club and pay membership fees. All the money, GEMA collects is then (minus a generously large administrative overhead) distributed among all members, depending on how big they are in the business. This means, small time musicians may actually subsidize the big names.
Well, you can't make a gift, then demand to be paid if the receiver (unexpectedly) makes money out of it. Better to foresee that offer paid services on top, e.g.:
At some point you realize that you have to ramp your business up, just so it pays for the cost of accepting money. Then it's no longer a side project.
I'll just leave this here:
https://raccoon.onyxbits.de/blog/software-development-cost/