I've got some experience with various databases, but little with
Access, sorry.
Given the OP exported the data from Access once, I'd assume that it's
possible to export to csv format instead of Excel spreadsheets, and I'd
recommend that. Then importing into Base should be straightforward, and
formatting shouldn't be lost. Access might even give some options for
the export formats, but worst case the data can be massaged in Calc
before importing into Base, say to change the date format.
If the OP is still having trouble with this, he can try posting a
couple of lines of the exported csv file, and we can see if there is a
reason Base is not importing it correctly.
This does sound like it should be possible with the method he is using,
but it's much harder for me to debug. If he's using plain csv files,
just posting a couple of lines would give me an idea of how the data
looks, and I could test on my machine, which does have Base, but not
Access.
And if he wants to go to MySQL/MariaDB/PostgreSQL, again using csv
files should be pretty straightforward, even if it does mean command
line and queries, and not a GUI interface. Although those exist, I'm
just not as familiar with them.
Paul