You may have better luck following an established program, for example check out the dumbell PPL program on the boostcamp app, you will be able to onboard using your current lifts, And it will provide you a progression path that you can use to make decisions about your weights. Maybe you could pick up some magnetic change plates or something stick to the dumbbells you have access to

If you stuck with it for a while and you're willing to invest, maybe consider power blocks. Personally I think you're primed to start barbell training, if your gym has a barbell then the sky is the limit

There are definitely ways to increase exercise intensity without buying more weights.

If your strict and incline chest press is at 50 lbs, what about decline press? You can also do hex press, single arm alternating press, and plenty more variations. Or chest flies, or the many variations of pushups with or without extra weight.

You did call out volume increases, but don't forget decreasing rest time or adding super sets. I have always been an avid and consistent gym goer. During COVID I transitioned entirely to body weight exercise and then a set of cheap resistance bands and managed to increase strength (though at a shower rate than I was used to with the near unlimited weights I had access to before).

As for bang for your buck. You can wait to find used weights online, or spring for a barbell and cheap plates. Even low end stuff (that isn't dangerously poor quality) will still run you about $1.5/lb at minimum. A barbell and a variety of plates are generally going to give you the most options for the price. Adjustable dumbbells probably being second, but only the expensive ones go much higher than 50lbs.

I hope that helps, happy lifting!

You need to go up in weight for progressive overload. You can get by a little longer through more challenging variations like the decline version of chest press, but eventually you'll want to upgrade