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!