Sorry to be the bearer of bad news, but your fretting hand technique needs work. You want your thumb behind your fingers (ideally positioned somewhere around middle and ring) and your hand and fingers perpendicular to the neck so that you’re fretting with the tips, not resting on it at an angle.
Another +1 for this.
Now, to speak briefly to some other points made here: sure, it’s absolutely possible to make really good music using “improper,” or at least idiosyncratic technique. For example, McCartney is one of my all time favorites. Still, l would say study his note choices, his bass lines, his compositions, his tones etc. His fretting hand not so much.
For me, the purpose of “proper” technique is to work with your body’s own physiology to maximize flexibility, strength, leverage, reach etc. I look at it as a center... you can drift from it here and there but you should return to it for the most part.
The purpose of the thumb behind the neck, in the general vicinity of the middle finger, about halfway down, pressing as lightly as possible, is to maximize how far you can spread your fingers. It’s kind of an ideal for playing
difficult stuff .
Where exactly your thumb points is less important, as it may drift as you play anyway. Mine is most comfortable between vertical and about 45 degrees towards the headstock. Of course, if I’m just chugging along on a 1st position C, l’ll probably have a baseball bat grip. At the other extreme, l might play the highest frets in cello position with my thumb in front of the neck. There can be a lot of variation.
Still, IMO, the more challenging the music gets, the more you will want to use a standard, “proper” technique. Or, at the very least, have those skills available to you.