The aftermarket support is always mentioned, but it honestly matters less than you'd think, as long as it exists in some decent capacity. The difference between having 100 parts vs 2000 is meaningless, if performance is the goal. There's of course some correlations between quantity and quality, but let's skip that for now. You won't be using more than one
specific kind of part at a time. It doesn't matter if there's 40 different variants of said part. You just want one quality replacement and that's it. The difference between biggest aftermarket support (Hi-Capa) vs. smaller one (AAP-01) is meaningless from perspective of performance. Both have access to the highly optimized quality parts.
But with that said, there's three obvious options that all make way more sense than anything else, and all of them have aftermarket support.
1) Hi-Capa. For obvious reasons. SSP5 is a good option here, which is honestly brilliant as a gun (you can dislike Novritsch for various things, but this gun is a beast). Can't be silent. A tuned Hi-Capa shoots laser up to 50-60 meters. Here's a genuinely well made SSP5 review
that shows what a Hi-Capa can do. SSP5 is completely fine as stock, but a regular Hi-Capa you will have to heavily modify to get great performance. Great for indoors and outdoors.
2) AAP-01. Very light, shoots well, bit toy-like with stock upper, can be tuned to be whatever you want. Advantage over Hi-Capa is the size and weight. And actual full auto, which is useful maybe at best 1% of the time when using a sidearm (and to be able to swap to it for those situations, you'll need to mod in a quick selector switch). Can't be silent. Tuned up will have similar'ish accuracy as Hi-Capa. Out of the box it's not bad, but also not great. Great for indoors and outdoors.
3) MK23. Silent. Accuracy is often praised, but assuming the other two options are modded, there's practically no difference between MK23 and them. Everything else about it is kinda bad, or the other two options just do it better. Trigger pull is pretty shit, and the mods to make it more tolerable have significant downsides. Size is ridiculous, especially the length. Rate of fire is bad. You absolutely need to mod it. As sad as it is, MK23 is still worth considering even in 2023. Just underlines how shitty the market is for NBBs when this is genuinely still the best option for a stealthy sidearm -- the others aren't even worth mentioning. 100% outdoors only, and even then only for a very specific use.
Personally, I wouldn't run anything bulky as a secondary. Mobility counts, as does space for carrying. HPA tapped Hi-Capa and AAP-01 also accept M4 mags, in case you've got a DMR setup for example. Which means you could save a lot of space, run the same mags on primary and secondary, and still have the luxury of having an MK23 strapped somewhere with a few spare mags. I'd love to get completely rid of MK23, but it's honestly really good for exactly that small niche it occupies.