I need to upgrade my gear, or at least the lens I use
Would make it easier, but not necessarily required if one is willing to work at it a bit. A little stealth and a well placed blind could probably get one some frame filling shots. A popup blind has worked great near desert waterholes and around places like burrowing owl homes, often with the critters so close one can not even fit their face in the frame.
Again a bit of work, but I've seen folks that put in the effort to disguise float tubes as muskrat houses. With slow approaches, they can get within single digit yards of waterfowl. Cookie is retiring in about a week, will have considerably more time, and will be taking on that approach next season.
Presently, she gets a lot of shots from her car by getting ahead of fowl along a narrow water course and waiting for them to swim into sight. Same approach works for coyotes and deer that are tracking fence lines or stream banks. One needs near everything in position, however, before they arrive, as just the action of lifting a camera to ones face is enough to spook a lot of critters.
This was with her 100-400 zoom at 230mm.
and at 250 mm
Best of all is just getting out there. Have a good one,