Initial import of configuration should immediately bring in IMAP folders (at least names)
When I imported a config file exported from K-9 into Thunderbird for Android only the Outbox folder was shown. After exploring some options in the gear menu (thus consuming a little time) the folder list still hadn't populated. I set the phone aside for a time and came back to it. The folders had been populated. I suspect that it may have waited the Folder Poll Frequency time period (15 minutes in this case) but I don't know this for a fact. Initial setup should retrieve the folder names immediately. This will provide feedback that the setup/import was successful.
K-9 6.804 Thunderbird for Android 8.0b1 Samsung Galaxy S21 Ultra 5G/Android 14/One UI 6.1
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Hey Dennis, Thunderbird for Android is still in Beta. The developers are ironing out some issues here and there. If you need more assistance, feel free to ask at https://support.mozilla.org/en-US/questions/new/thunderbird-android
Yes, I'm aware it's in beta. I assumed that reports of issues would be useful to the developers.