On the laser option.
A while back I found a small flex head LED flashight that came with a red spot laser "cat toy" in the other end. Price was a whopping great $1.89 or something like that. I bought two.
The flex head is superb for lighting up barrels for inspections and the laser, when not amusing the cat, would be easy to set up for this sort of trigger pull dry fire practice. The whole thing is not much bigger than a basic ball point. So it does double duty by coming along to gun shows with me to act as a bore inspection light.
Balancing a penny on the end of the barrel would be good too. But if you're alone then just trying to put the penny out there and then get your hold and all without it falling off would likely try the patience of Job hisself....
For this sort of dry fire trigger control practice I like the idea of the laser far better.
Note that the best results will be seen when the laser spot it out a good 5 yards or more away to magnify any shudder or pulling. Down in the basement aimed across the whole length of the house at one of the concrete foundations is perfect.
As for your shooting given the description of what you've got, how it's set up and the ammo you're shooting I'd say you're actually doing better than what I'd expect. You've got so many things working against you with that overall setup that it's not funny. Correcting all the aspects you've already read about will make for a huge change. Just using a soft rice or barley bag up front will help a lot. And some rear support of some form is another key along with modifying something to attain a better cheek weld.
A trick I was taught for bench shooting with only a forearm bag is to set up the forearm bag such that you can put your weak hand back under your arm pit of the strong hand. Making a fist with the weak hand and using that as a sort of variable rear support works not too badly.