Not who you are replying to but I always fell back to a single monolithic inbox with categories/labels as the differentiator (professionally and personally).

For me, this was down to my line of work where client projects would be anywhere between 1-6mths, each with a revolving door of stakeholders. If I had the time during mobilisation, I could set up a system but it would just take one particularly active day or brief holiday worth of incoming for it to no longer be managable - resulting in my emails now being in two places making it annoying in time critical situations, and easier to miss mails generally.

Lastly, less of an issue these days but I used to always run into search issues when everything was segregated into folders. Sometimes this was due to early 2010s online inboxes still being anemic in size (and forced to offline archive) and, sometimes I think it was just old software creaking. Filtering by tag/category/label was always functionally the same number of steps for me but yielded better results - and for incoming, visually seeing a brightly coloured label in the single list of mail draws my eye more than a small “(1)” in a sizeable folder structure.

The first thing I do when I start at a new place is create a mark “X accepted your meeting invite” mails as read & auto-archive.

Immediately cuts out so much noise, and you will still get any Tentative/Declined in your inbox. Always the first thing because at the beginning you are likely setting up lots of meetings with new people.