The conversion during import is not necessary with the appimage version, I know because I do it every time I rewrite my machine.
I just download the appimage version of thunderbird. and befor I start it for the frst time I copy over the entire .thunderbird folder to my new home folder. Then start the appimage thunderbird. Because there is already a .thunderbird folder there, it does not try to make a new one. Instead, it uses the one I have there and loads it up in Thunderbird on first launch.
Edit to add.....
Actually, scratch that...
I have just taken a nother look at the version I am using. It seems I copied this last one directly form thunderbirds own site and downloaded one of the legacy ones. Can't remember why, but it might be related to what you are mentioning.
As it turns out, this is also standalone, however. Just download and run the binary in the folder like you would with an appimage. I also recall, upon first loading it, to immediately go to preferences and turn off "updates" to stop it updating. Again, I now recall this was to stop it buggering up and not working with my .thunderbird folder.
Memory is wonderful thing, I seem to remember.
It's a pity I can remember bugger all else...
The legacy version I am curently using is:
https://archive.mozilla.org/pub/thunderbird/releases/45.8.0/linux-x86_64/en-GB/