SQLite. It's not close in terms of ease-of-use of the GUI administration and forms, but as a single-server database solution to embed into a line-of-business app it's fantastic.
It's a real shame Access died, now there's no path from a spreadsheet to something more sensible.
Although it has certainly died in terms of mindshare, it's officially still supported for PC (not Mac):
https://www.microsoft.com/en-us/microsoft-365/access
I actually have it installed right now, because it comes with Microsoft 365 Family, which I have on the Windows PC on which I'm typing this comment. I haven't used the modern version of it, though.
If your application grows into the need for concurrent writes, then it will be time for another database.
There are basic SQLite GUIs, but they have no reporting functionality and no customizable forms.
A cross platform Access-like front-end that used sqlite as the data storage engine would conqueror large parts the territory.