I have been fielding questions about this product that I personally loathed back in the 1990s for much more than decade. I have also upset a lot of Eudora folk that will not accept that Eudora has some major weaknesses and this is one of the major reasons importing from it is so prone to failure.

Before you enter into using Thunderbird there are some thing I think need to be crystal clear.

1. Thunderbird is not Eudora. Things are done differently and layouts are different. If you think you can make Thunderbird look like Eudora you probably need to go off and have a look at the resurrected Eudora project using the source Qualcomm released.
2. Changing the default directory on a per account basis, while offered in account settings is a recipe for future failure. Do not do it.
3. Do not try and place mail data in your documents, pictures or other system folder or the root of any drive. There is a high probability you will loose data when your antivurs of Microsoft backup corrupts the mail store.

Do not meddle in the Thunderbird profile folder with the mail storage. Again data loss is the most common outcome. Even I do not touch that stuff with Thunderbird running in the task manager. (the absence of a window does not mean that application is not still running)

Thunderbird stores mail in profiles. See https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data it is via the profile manager you change the drive to use and the folders. A full location for the profile structure is what you set. But I do not recommend doing so until you have your data imported.

Do not install the Microsoft Store version of Thunderbird if you are gong to change the profile location. Personally I think it is poorly tested and no one really has the details of what the windows sandbox for store apps does to Thunderbird. They only think they do. I have seen no explicit testing involving the store version,m just an anjnouncement5 that it was released.

IMAP essentially does not have a local store worth mentioning. It caches copies locally, but as soon as there is a problem verifying the local copy with the server the local copy is deleted. So do not get caught.

The official docs, such as they were, were basically deleted when Qualcomm abandoned the Eudora OSE implementation. There are none and as you are over 10 years late getting to the party, there are few that actually were involved in support back then to even offer ideas. Thunderbird does not have any Eudora import code any longer, that has been purged from the code base probably about V34 f I recall correctly. However if you get an old enough version from here https://ftp.mozilla.org/pub/thunderbird/releases/ Perhaps V30 or 3.1 installing it on a system with a functional Eudora will see things just imported. Then you can update to a more recent version as those old versions do not work with major mail p0roviders any better than Eudora does. Mostly with regard to Oauth authentication and TLS encryption bnut you will not be able to get mail from Yahoo, Hotmail or Gmail with V3.1 any longer I do not think.

This information https://kb.mozillazine.org/Importing_from_Eudora_-_Thunderbird is still Germaine and the reference to Eudora rescue is often very relevant, not that I know if it is available anywhere on the web any longer. Despite a loyal user base, Eudora was really sloppy with it's stores and tolerant of errors caused by incorrect file edits. Something that sort of goes with an application that encourages manual file editing. Nothing since has even considered accepting a Eudora level of inherent corruption without major issues.

I wanted to thank you quickly (now) for your reply. There's a lot to digest there so I'm going to take the time rather than peppering with a bunch of not-well-thought-out questions...  :) tnx, tom

I did not want to be rude, but this is a process that is a mess. A lot of the blame falls with lax standards in Eudora and Eudora fans do not accept criticism of their chosen mail client well. They profer Eudora and have been determined in the past hat Thunderbird had to be changed to do it the Eudora way.

The longer after the demise of Eudora that transition occurs the more they appear to expect a robust well documented process. The reality is it is about exactly the reverse of that. The less folk other than them that even remembers Eudora.

So I come in hard and somewhat aggressively in the hope of defusing the agro that goes with folk hearing "No" in this world today, where even facts can be "alternative".