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I skimmed past the bits about sites that monetise users (and this would be that), but yes it introduces a difference.
If you changed nothing at all cost-wise today... £1 per person per month would be enough.
But... not everyone donates, at peak only 300 people did, and PayPal and other payment providers will take their cut (20p or 5-10%, whichever is greater or something like that)...
So the ideal is something more like £10 every 3 months, for ~250 people to yield around £800 per month, and therefore always have a little more being accumulated such that you have a buffer and if you ever need to add a server, there's the money to do so.
You could keep it donations based, no paywall, if ~250 people signed up to a payment structure like that.
This was what I always aimed at, but as people's payment methods expired, etc... well, I just made up the difference and didn't both to do a focused fundraise in recent years.
If I were doing this now, I would 100% set up an Open Collective https://opencollective.com/europe most bills are in € or $, and I would have someone pay the bill on a Wise card, and then be reimbursed from Open Collective... with Open Collective taking the donations, and showing how much is in the bank, etc... the transparency I wanted to give, but couldn't do via PayPal.
It does look possible to have "officers" in other countries, the servers and systems all over the place, the money running through OpenCollective EU, an entity in US/France/Switzerland... and only volunteers and users in the UK.
Note: even in this scenario... I would step back and fully yield all control. For a collective to be successful, I should reduce myself to an advisor at most, just to point out how things work technically, how situations were approached, etc.