You may not need a special long-hold remote at all. On both the Nikon D7500 and Panasonic FZ300, bracketing can be set so one press triggers the bracket sequence.

• Nikon D7500: set your bracketing with the BKT controls, then use single-shot release mode. You can also use the interval timer so one press starts the programmed bracketed shots automatically.
• Panasonic FZ300: in the Auto Bracket menu, set it to Single so one shutter press captures the bracketed sequence.

In that setup, almost any remote release that simply triggers the shutter should work.

If you specifically want a button that can be locked or held for long exposures/bulb use, a wired remote is a simple option. Basic wired releases often have a two-stage button and a lock, and extension cables can increase the usable distance because the remote is only closing an electrical contact.

So the best answer is: use the camera’s bracketing settings to avoid needing a long-held press, or use a wired locking remote if you truly need continuous hold behavior.