Hi there!
I've been using that mod for quite a bit now, and while enjoying the Janissary quite a lot, that said I can't help but feel like he is pretty... "Unremarkable" as a back liner.

Here’s the problem I see, his “gimmick” to so speak is that he needs to stay at rank 4 to enjoy “Brace the Gun” benefit, but other than that, is standard attack “Arquebus Shot” is fairly unremarkable while Thin the Herd does 80% damage split in two, with some interesting but not amazing debuff associated to it. Song of Valor is interesting tho as a filer skill to shave a bit of stress at the end of a fight.

So in the end, you end up with a backline that rely solely on Arquebus Shot to deal damage and Brace the Gun to gain a decent (but pretty niche) buff against armor and some CC and crit (which is nice, but again, not ground breaking) under the very strict condition that you need to stay rank 4 and not move from there (especially since you don’t have any Back moves).
So here’s my suggestion to try to make him a lil more attractive as a back liner while making him a bit more unique and maybe even give him a nice nich to fill.

First, I’d add to his Arquebus Shot the following : “Ignores Riposte if in position 4.
From a design perspective it gives him a role that no other back liners have, reinforce is Rank 4 design and make sens flavor wise (he is literally sniping from the back of the line, pretty hard to go riposte against that).

The second change I’d like to suggest would aime at making Brace the Gun a little more reliable as a tool that would help you sit at Rank 4.
Here’s what I’ll suggest: to add to Brace the Gun “Becomes Immobilized”
This would help him stay in place and not get so easily knocked off his precious rank 4, while reinforcing the whole idea of him putting his arquebuse on a stand.

This would be the “simple” change. But I think we could go even a little further (but that would require more works, so just consider that as a “would be nice but not necessary”.
The idea would be to make him change stance after using Brace the Gun (literally changing his idle animation to match his pose when he uses Brace the gun), and make Brace the Gun have a different effect when already braced.

The skill would then look like that :

Brace the Gun
When Mobile:
Self :
Becomes Immobilized
Armor Piercing: +X / +X% CRT /+X ACC while immobilized
When Immobilized:
Back 2
Self :
Dodge +X / SPD +X / Stress -X

This way a back liner Janissary would be able to not only stay in the back where his spot his, but also, if needs really arise (like if someone else is knocked back behind you with a stun or something, he will still be able to redeploy in the back by reusing the skill, losing a turn (2 to be exact, since he’ll need to redeploy again), sure, but gaining something out of it.

As I said, I know this version is way more complicated (both in the sense that it will need a new idle animation but also a new skill) but with that new wording (closer to the Graverobber’s Shadow Fade) you end up with something that gains a bit in readability.

Also, a last point I’d like to make a suggestion for if I can : While his frontline kit is way more interesting, is Tempered Slash feels very… vanilla to say the least.
Here’s a small change I think would not only reinforce his Guard focus, but also, once again, could give him a new nice that would help to make him more attractive as a pick in some situation :
Simply add “Ignores Guard”.

And that’s it. He will be able to catch a kill that the rest of the team could from time to time, and it will feel good without being too powerful overall.

I realize now that was I wanted to be a small suggestion ended up in a pretty big wall of text and I'm sorry about that, but I hope you'll find something of use in there.