In context of "Swapping the 2 power wires for the decoder should be easy, " I was imagining a loco with no dcc decoder, no socket, old style, but assume from your reply old 4 pin decoder (sockets) are replaced for later versions with more pins.
"Don't limit yourself to one manufacturer. "
You are right. I have to-date on basis as same make equates to fewer problems.
Now I seem to be confined by availability of 1 m/f when looking at DCC options, also, I seem to have opted for GWR Region, so need to cast net wider for variety of rolling stock.
>0-6-0 Pannier tank locos generally don't have space for speakers even if a decoder can be fitted.
Well those big panniers on the side need to store something.

I've seen videos of them with sound, but since seen that it was sometimes a post-production item or O guage - need to look some more I thought I'd seen them.
I suspected there isn't space, as I've seen photos with big locos having to use tender for "extras".
This is where I believe m/f need to re-visit to accommodate speakers into the design and enthuse the buyer rather than expect each user to have technical skills to retro fit.
At present, it would seem the advantages of DCC cannot be fully used due to not having a cavity at design stage.