You’ve got Molle/pals webbing on the popup belt so a holster setup for that is a no brainer. I made up an H harness out of webbing for a belt holster so that I can retain a holster on my guidlight belt which doesn’t have molle.
I carry a 4" N frame S&W in an old Hunter holster in the cross draw fashion on my popup 28. It slides over the entire belt allowing you to carry it toward the middle of your waist for easier draw. I think the molle webbing on that belt is to far back for a handgun.
Are you handy at all? This was my 3rd or 4th version of a chest rig, super easy with almost any exhisting holster. With some kydex or warmed up pvc and some clips this took all of 10 minutes.
It's a swaprig holster. SwapRig Holsters - Love Your Rig... Swap Your Carry!
You buy a chasis and the custom molds that bolt on are like $15 each. Can go iwb or out with the belt loop, comes with extra clips. It's super versatile and I love it. However if this is the only use (and you're a little soft around the midsection like me) maybe find something else. My gut occasionally would pinch a bit between the corner and the waist belt on your pack, not a big. Pretty minor though. I've got an XDS but opt for the 12rds of .40 more regularly.
It was my first time going chest and I'm never going back. Easier to access, still haven't if you take your pack off, you'll know if it falls out, more room on waist for pouches or whatever.
So my suggestion is find a holster you like, a few straps around the house, a couple buckles and a 1' square of kydex and you can make it however you'd like and save $100 or so.
If you're more the type to just buy something, I've heard these are good. Kenai Chest Holster – GunfightersINC
I use the biabchi military holster with my Glock 20. It's nice and easy to move from belt to belt. You can use the thumb release but I run the flap for most of my uses.
Have you considered a Safariland holster coupled with their QLS system ? It allows you to put adapters on a number of places and them simply detach the holster from the adapter and move it as necessary.