Hello,
Could you explain the advantages of using the figured bass tool rather than the lyrics tool?
I came across the following web page:
https://tobis-notenarchiv.de/wp/bach-archiv/vokalwerke/messen/.
In this collection, the figured bass has been entered as lyrics.
I am wondering what benefits the figured bass tool offers over the lyrics tool, particularly when it comes to further editing these files.
These files are musicxml 2 (the current spec is 4) and I don’t think figured bass was defined then.
Whether figured-bass it is encoded will depend both on the exporting program (in this case I think Capella) and Dorico.
With the native figured bass in Dorico the figures will change if you change the bass notes. To be honest that is something I never found useful, you will have to judge for yourself.
Another difference has to do with extension lines that will work differently. Some people who need features not covered in the Figured bass function (for instance partimento notation) will also use lyrics instead, for instance with the Go Figure font by @benwiggy.
It would be nice if there were a tool or script that could convert lyrics into figured bass.
You mean it would be nice if someone would write one! 
There is still no mechanism for Dorico’s Lua script to query document data, e.g. to obtain the text of a Lyric syllable. (To say nothing of that fact that every font used to render text as bass figures has its own syntax.)
I’ve retyped the bass figures in all my older Dorico documents that used Figurato – it didn’t take very long, and it was a nice opportunity to check it over.
I compared the lyrics section with the figured bass section in MusicXML.
Converting lyrics into figured bass is not straightforward, as the figured bass entered in the form of lyrics often contains more than one symbol, sometimes two or three, with specific rhythmic values. I therefore have to convert them manually, comparing the theme with the original score. It is indeed quite a task…
If the imported lyrics figured bass works for you, just leave it as it is. Maybe there is no practical need for you to convert it!