It is quite natural to think with tags intead of folders, as one email might be part of multiple "topics".

E.g. if I receive an email with the invoice from Proton, I am might tags it as:

e-services/proton
expenses/communication

I want to see the email in all searches:
- e-services
- e-services/proton
- expenses
- expenses/communication

Gmail does very good job with subtags in emails, it creates an artificial tree structure. Check also how it is done in Blinko app (https://blinko.space/).

Create a conversation archive, with assignable TAGS.

If you look the sieve filter syntax. Folders and labels are effectively the same.

Mail can only be in one tagged folder but have multiple labels.

So you could have a filter that moves email from an airline address to travel folder and label it travel.
Then you could have a second temporary filter that also labels these Hawaii. (plus even move them to a sub folder)

Custom seive filters are powerful and chatgpt, claude and even protons lumo can help write custom sieve filters.

Given that emails that get threadded together end up moving between folders as a group, this is pretty important for organization.

I agree