Well, there are currently 142 Tonies and 100 custom tags - and I have enough content for another 100 tags, and counting.
But then I’ll order another card.
Incidentally, that was the reason I asked in advance whether there was any information about which cards work with the ESP32 or not. But apparently there isn’t much feedback on this yet. I would put this one in the “or not” category ^^
But even with this numbers. 300 * roughly 50mb or be generous 100mb = 300*100mb=30.000 MB = 30GB
More like 200 - 300 MB for the custom tags, as I have either long audiobooks or up to 10 episodes of a series on many of them - the largest ones currently have over 400 MB, but that’s actually the exception.
And original Tonies are still being added, but of course they are correspondingly smaller at around 50 MB.
As I have little desire to adjust the memory again at some point, I will probably install 128 or 256 GB. Incidentally, that was the reason for choosing such a large memory card right away 
Different card, same result…
A 128 GB San Disk that I still had here. Same formatting, image and image extension as described above - same result: It forgets the downloaded content almost immediately. If I download two Tonies and a custom tag, I can immediately start all over again…
What’s going on here?
Edit:
No idea if this is helpful in solving the problem (or obvious to those with more knowledge than I have anyway), but when I clone the original card, the problem still persists as soon as I download new content. The existing tonies and custom tags behave as they should.
No matter which of the two cards (512 GB Samsung / 128 GB San Disk), it pulls the content and plays it offline when I switch to offline mode immediately afterwards. However, if I turn off the box, the content is no longer available the next time I start it and it either complains in offline mode or downloads it again directly in online mode.
This time I cloned directly from the original card (with EaseUS Partition Master), so it makes no difference whether I use the existing image or the card directly. The problem remains the same.
If I format the card again, it always tells me that the existing partition is write-protected, so I have to delete it first - is that perhaps the problem? I don’t set anything like that, so it must be write-protected somewhere unnoticed when cloning/imaging. The original card obviously does not have this problem.
The problem seems to have been solved - I have now only formatted the 512 GB card to FAT32, no image and no clone of the old card on it. Now it seems to work, I will test it again tonight.
Whatever went wrong with the image and clone, that seems to have been the problem and not the SD card itself.