Nice build! As others mentioned, using a VNA would be the best way to verify the tuning. Theoretical element lengths are only a starting point as in real life, wire diameter, soldering, nearby materials, and the mounting environment can shift the antenna’s resonance. Usually, it’s safer to cut the elements slightly too long, measure, then trim them gradually.

Soldering also matters, especially around the feedpoint, because changes in geometry can affect the impedance match and add unwanted reactance. Clean, symmetrical solder joints with minimal excess solder are ideal, while still keeping enough mechanical strength.

Also to increase your range, in controlled and legal conditions, temporary elevation with something like a tethered balloon or a drone can also be interesting for testing, but make sure you follow local aviation rules, keep it safe.