I think I will stick with POP. At present I am gradually sorting out Thunderbird taking my time by transferring folders on Outlook to Thunderbird one at a time after clearing out the folder on Outlook of emails dating back several years.
POP (Post Office Protocol) is a protocol from the stone age (i.e. MS-DOS) and as I recall pre-dates even http. IMAP is much more recent and better at coping with the much greater range of requirements such as dealing with multiple devices accessing the same email account -e.g. PC, laptop, tablet and phone.
I have 3 personal email accounts (my main one with a personal unique domain name, a hotmail one, and a gmail) plus a shared gmail account for a sports club service for which I am responsible.
The difference with POP is that the individual email items are downloaded to you local mailbox and removed from the server. IMAP is a server based store which synchronises with any of the signed in devices. So, with a POP mail, you login to your email on a PC, that mail disappears from the server so is not available to any of the other devices connected to the account. IMAP has a mirror copy of the mailbox on each of the devices which means that the item is still available to the other devices unless one of the devices actually deletes the item.
I manage my 4 mailboxes through Outlook (again a user since Office version 4,3). My desktop PC runs a mail washing software which deals with spam - moving it to the spam folder on each account - and also manages Archiving.
One of the things people need to realise is that if you just let mail pile up in your mailbox, it gets bigger and bigger, and loading takes longer and longer (my wife has AOL mail that includes messages that are 20 years old).
So, with archiving I set it to run daily, and delete items in most folders when the item is older than a month. HOWEVER... there are some emails you don't want to delete, so I set up a Subfolder in Inbox called Saved Email, and have now divided that down with specific folders for (e.g.) Insurance, car, car insurance, holidays, etc,etc,etc.
My main mailbox is 191MB... of which "saved" is around 33MB and the remainder is roughly what 3 months mail looks like.
Hope this helps...