Please check out the linked thread, which this question will probably get merged into.
I’d start out by echoing @Craig and recommend an entry level pair of planar headphones. Since his favorites by LCD are no longer available, I got the Hifiman HE400se. They are easily the best headphones I’ve ever owned, a big step above the AKG K240 Mk2s which were probably my best prior headphones. The AKGs are good, but the Hifiman headphones are better, less expensive, and seem more robustly built. My AKGs failed when a plastic piece that connects the metal headband to the earcups failed. Plastic isn’t necessarily a dealbreaker - high performance polymers exist, but AKG doesn’t seem to use them.
I’m personally reluctant to spend more than $300 based only on recommendations (exception: Sony WH-1000XM4, but I bought those for listening, not production or mixing. Also, my last employer paid for them as a WFH expense). My process was to buy the HE400se and get familiar with them. I can’t find any faults beyond the very short cable (good for listening at the computer, less good for synthing), so I’m unlikely to upgrade to the more expensive models any time soon.