At the same time I think there exists an opportunity to bring the ordering and structuring function of transactions to the open source marketplace.
And I don't think the donations are subscription model really works for the reasons you enunciate meaning that everything is too unspecified, yet there are definite implicit expectations of some sort of transaction or exchange. The non-specified nature contributes a lot I think to inefficiency as well as to stress for both sides of that marketplace. It's only one of the factors that makes changes in open source currently not ideal.
With current models, even if you do specify it it acts maybe in a sort of way like a patreon model which doesn't really make sense in open source which has almost unlimited number of discrete continuously redefining tiny tasks.
The way I see payments working is more as a formalization of your choice to say yes or no to things so basically like okay here's sort of like my minimum fee or here's what I'm quoting to take a look at this. There's no guarantee that you know the entire issue that is unspecified and that could balloon into multiple future directions is going to be solved by this but here's the amount which I need right now to take a look at the next step.
Some people might view that as a way for them to make money or extract value or get reward for themselves out of their projects that is sort of tied realistically to the actual demand that people are prepared to put their money where their mouths are.
Whereas other people might see it simply as a way ensure they do not get overwhelmed by allowing the market to decide priorities, and provided down regulator or gatekeeper on the amount of incoming work. The filtration function happens before the requester even makes the request ideally because then once the expectation for the transaction occurring sometimes is established requesters may think more about steps they could take themselves other than operating in a more depending on demanding way. Possibly anyway.
For the creator, you can formalize your no by putting a price in front of it saying it'll cost you this much for me to take a look at it. But the thing is the person who made the request isn't the only person looking at that issue presumably and they don't have to be the one that pays that through.
Anyway I just just an experimental idea. The real interesting thing and where the meat is here is like how do you structure these sort of micro transactions, that sells problems on both sides of that marketplace. In other words you're allow developers to exercise more control over their incoming workflow which is a key driver of happiness you provide the opportunity for them to get rewarded for their open source efforts in a way that's actually tied to specific toss rather than a nebulous sort of funders model or sponsors model, and you provide a bit more like clarity and definition for requesters you know to think well is it really worth it to me to to make this request and how much am I prepared to kind of you know put up to have this creator I take a look at the next step for me.
I think there's multiple ways you could configure bringing the ordering and structuring function of transactions to the open source community. The above is just Possibility.