how to proceed? templates and instruments for future pieces?

Greetings,
I have a question, please.

I am still somewhat new to Musescore. Actually I have scored a lot of pieces, but I am still struggling with some concepts. If I want to create an instrument with multiple channels available, like the following:

https://musescore.org/en/node/24410 (scoll to the bottom comment by Jojo)
https://musescore.org/en/node/125716#comment-643946

Plus...I also commonly score out keyboard parts for piano, organ, synthesizer, mellotron, etc.,
but I always arrange these for two hands, two staves. Hence, more instrument changes.

How exactly do I save said instruments for future use in other pieces? Templates? Instruments.xml?

Please keep in mind I cannot always save an entire score template; layout will sometimes change for each piece. I have gone into the mscx files with notepad and added channels, but that is it. This, of course, saves it for that piece only. I have been leary of changing the instuments.xml file.

If I tweak one file for just guitar until I think it's perfect, how should I save it? Is there a way to add it with all channels to a song already created?

Any specific help would be greatly appreciated, including any parts of the handbook I might have missed.

Thanks,
mkj