I'm fairly certain I know exactly what will help you drive through with effortless power, and why you feel like you can't do that right now.
First, a point of reference. Once you really get the mechanics down, you can crank as hard as you possibly can with your legs and abs, and still make most balls (if they're sitters). I'd wager
good money that when you try to hit as hard as you can, you cycle between hitting the net and hitting it long, or that you just can't seem to actually hit a 100mph forehand no matter how much effort you put in, because the transfer of force from your core, through your wrist and hand, into the racket just isn't efficient enough.
Here's why - the volitional backswing. (aka the stuff you're doing on purpose with your arm and hand muscles, not what's happening as a result of flinging the racket forward with your core)
You are extending your elbow and rotating your strings towards the net with your hand, rather than allowing those things to happen passively as you drive the racket forward with your hips and abs. This is why you feel like you have to be so careful - if you relax and swing out, you're definitely going to hit the ball long with this preparation.
Here's how you fix it:
1. Prepare palm down.
2. After you place the hand down, don't do anything else with the hand. Just rip it forward with the torso.
I think Aslan Karatsev is the player whose forehand best illustrates what I'm talking about:
https://faulttoleranttennis.com/compact-efficient-elite-aslan-karatsevs-versatile-forehand/
Check out his preparation. He places his hand down, palm down, and
that's it, preparation over. His next step is to explosively rotate forward. Because he starts palm down, he can control the ball with a relaxed wrist while exploding. It is because the strings will always turn up as you explode forward that you must start with them facing down. If you start with them pointing at the other side of the net, you'll be forced to either swing slow, or use wrist tension in order to avoid them turning up during the forward explosion.
Oh, and btw, your forehand is pretty great. This is probably the one tweak that'll unlock everything, but overall the fundamentals are very solid.
Also, you are correct that a stronger left arm will improve your forehand, but marginally. The better symmetry between your elbows as you swing - using the entire upper body as one unit - the more you can rip the ball with your ab muscles while staying balanced. Your left arm is fine though, that change is maybe +10%. Fixing the string angle during preparation and removing the volitional backswing will fundamentally transform your swing. With the amount of effort you're using now, you'll feel like you literally can't miss, and you'll also be able to absolutely rip the ball with 80-90% accuracy, because the rest of the motion is already so clean.