First of all... get rid of your mask mount
. You most likely wont be able to get steady footage while pointing in the right direction.
Get a pole or maybe better a tray. I use a pole and won`t be changing that because thats what i bought first (cheaper) and won`t shell out more money.
I might rather upgrade to a bigger camera rig. Key thing is good buoyancy control, slow movements and calm hands!
The pole has some advantages though.... it gives me some room to marine life and i don`t have to come too close which might scare some creatures on OC.
As for modes... for me i found 1080p@60fps is the best allrounder. If you want to get super slow motion shots you might go up to 120fps but only in good lighting conditions.
Same for 4K... you have to have very good lighting conditions and you can only go 30fps which in my opinion is insufficient for underwater footage. You will want that 60fps to smoothe out movements, fast moving subjects and for slowing things down when needed.
Shoot either in narrow or wide mode because u wont have that much bending around the edges of your footage. Stay away from that.. super wide mode or how they call it. Don`t exactly remember how it`s called cause i dont`t use it
.
Filters and lights is a whole new discussion but feel free to ask if you have any further questions.
These are roughly the basic that came to my mind. Most important part... practice, practice and again practice
This should be a starting point which hopefully will get you there. Hope i could help out a bit. For some examples you can checkout my YT channel.
Greeting Chris