You can always use Wingits like these https://www.homedepot.com/p/WingIts-World-s-Strongest-Fastener-Standard-6-Anchors-RC-MAW35-6/202727991 to mount to drywall. They support up to 300lbs.
You could use Wingits drywall mounts (like these WingIts). They hold up to 300lbs. I've used them to install grab bars in bathrooms, a water fountain to a wall, and multiple 70+ inch tvs. Obviously a stud is always preferred, but sometimes it just doesn't work out like that.
As for the OP, a 2x4 across known studs would probably be the safest bet