Regarding embedded databases in Base, there is some strong advices against using them when creating a new database for critical-data management, because it seems to be prone to generate data loss (see the references below).
Considering this, maybe the 'New Database' wizard should provide an option to let the user choose between an 'embedded' or a 'split' database. Probably, selecting the last option would not lead to the creation of a single database file (.odb), but a folder containing all the necessary files to work with the database, and to make it portable as an .odb file actually is.
Some documentation about how to afford it from the user side can be found here (maybe something here can illustrate how to afford it from the developer side):
Unfortunately, this 'workarounds' have some limitations with the current LibreOffice version. Specifically, as you can read in (4), the table designer is unable to modify fields properties after the fields has been created. Additionally, there are some problems when you try to set default field values for several data types.
Finally, this enhancement request is probably related to bug 61972.
Confirming as RFE