I have also always found this feature very annoying, as well as useless. What is it actually for? You usually want/need to change the instrument names and abbreviations anyway (unless you like these pre-programmed instrument choices, which I dont ...).

I think the best you can do is to have a template document that has all instruments set to Blank Staff, with your Device, Staff Names and Sounds alreadty established the way you want them. Then, depending on your score needs, you can just delete and move staves around as you wish. (And, if it really bothers you, you can also drag the Instrument column all the way to the left so you don't have to see it ...)

But it would also be nice if one could just edit this Add Instrument list somewhere, so one could only see what one wants.

I'm not understanding the problem. If doing it once only, then use Score Manager and change everything as you already know. If you are always going to be doing this, save your score as a Template. You've always been able to do this in Finale. Templates exist to let you do exactly what you want here.

You can also do both. Create your layout and Save. Now reopen. If you want to make some changes for a template such as generic renaming etc.,  use Save As and place the copy into your Templates folder. It will want to save as the original name with the word, copy appended but you can rename the document at that time or even keep the same name since you are placing this copy into a different folder.

The Help files aren't very uh… helpful here stating,

Once you've set up your score, choose File > Save As. Name the file and save it. By default, Finale templates are saved in Finale 26/Music Files/Templates, but you can save your templates anywhere..

https://usermanuals.finalemusic.com/FinaleMac/Content/Finale/ht-templates.htm

True but not quite accurate. I'm on a Mac. The very first time I tried to locate that folder in the drop-down, I couldn't. From Finale 26/Music Files/  I had to Create a new folder and name it Templates, then save mine inside. I was worried that I'd have a hard time accessing Templates from the New/Document from Template… menu but no, there they were plus the one I created. For this example, the one I created is called Tab Template and when I open it, there are my new instrument names as I created them.

Now, I can find this folder when I Save the first time or Save as… later.

I don't know if this behavior is the same in Windows. It's possible I found a bug.