Any advice on holsters, lights, red dot, extra mags?
Yes. You will definitely want a light. Before I never carried a light. Now I will never carry without one. It's just too valuable of a tool. Even if you don't know why or how, buy a couple of quality lights that have nothing to do with carrying a pistol. In my opinion a light is one of the most valuable and used tools that I EDC. I carry no less than two decent lights with me 24/7, three if you count the weapon light.
It's important to have a seperate light because if you need to help change a tire or look for a lost set of keys you should not unholster your weapon for something like that, especially when you can point it at someone that doesn't need it pointed at them.
As far as weapons lights go that too is a non negotiable thing now for me. I am still learning a lot and recently I acquired a 22 pistol that I put a light and a suppressor on. I can shoot that all night without alarming the neighbors, and hence I can practice and learn a lot more low light stuff.
Learning how to properly use a light is a whole other can of worms. It is in my opinion one of the highest priorities outside of just safe weapons handling and proper shooting techniques.
As for modding out a carry gun, I would keep it limited. I DID mod mine a little (I put in a new trigger and springs), but if you mess with the internals be ready to shoot it A LOT to test it. I put no less than one thousand rounds through mine (post mod) to ensure proper function. If it's something like a different grip module, still test it, but if you mess with the insides, test it A LOT.
I am also now a huge fan of red dots. That for me, now is also a requirement, or at least a really strong preference.
For me any carry gun needs to also be 'shootable'. By that I mean if you are going to the range with friends for a fun day out your carry gun should be something that you are eager to shoot. Getting a tiny gun with a six round mag that is uncomfortable to shoot means you typically won't shoot that gun, at least not enough to get decent with it. For me, so far the XL is small enough to carry without feeling like I have a boat anchor attached to my belt, but it's also fine for going out and shooting a couple hundred rounds with for fun.