Hey guys, been awhile, blame winter lol.

I decided partly out of boredom to try and get Mitzi's speedometer working right, it reads a little high but that's not so much of a problem, at lower speeds 20mph-ish the needle will bounce and sway and at very high speeds like on the highway it'll do it too. It's particularly annoying at low speeds because you can hear the needle clicking inside there as it's bouncing.

I got the cluster taken out and taken apart down to the bare bits, but the actual speedometer mechanism is much more simplistic then I expected, I figured there'd be a worn gear or something obvious but no. A guy I watch on YT named Rpeek replaced a white plastic gear on his 55 to fix his problem but now I'm guessing that gear is actually on the transmission side probably.

How have you guys fixed a bouncy inaccurate speedometer?

This is a PASS CAR with a cable driven speedo?
The cable itself may be dry in the housing and needs to be cleaned and lubed. The driven speedo gear is at the end  of the cable (trans) and the driving gear is on the trans output shaft.

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If you have the cluster out, simply and gently pull the inner cable out clean it with brake cleaner or equivalent, lubricate with Lubriplate available from Napa, I fill the cavity of the outer housing and spin the inner cable into it,if cable doesn't spin easily check for kinked or worn spot on outer cable.

My 55 was doing the same thing.  I lubed the speedometer itself.  Use very light weight oil, small amount on the brass fitting that the cable goes in and if it is on yours a few drops on the wick going to the inside of the housing.  Ford used two brands of speedometers then and I'm not sure if both had an oil wick.  As long as you have it out of the dash, you can get to this easy.
Ben