How to polish an old dull fiber shower+tub combo?
Here's what I did, while it doesn't give you the brand new tub experience, it is much cheaper and faster than a tub refinish, considering that DIY refinish is a pain and possibly a bad investment vs professional services($$$).
First clean with Bar Keepers Friend. Then dry everything
Next, use car rubbing compound with electric rotary polishers. I tried rubbing with my hands and it was not close to cutting it on the tub.
You can use different types of polishing pads from coarse to fine, and that really  gets the surface smooth feeling as well as get rid of some of the harder stains. It also helps with light scratches.
Finally, refinish with a car polish spray, it keeps a fairly waterproof finish on the surface to keep the tub cleaner longer.
The tub is finally presentable and a delight to bathe in, it will look clean and feel smooth, but perhaps I slacked a bit on the polishing, it doesn't have the shine.
Hope this helped someone who needed an in-between solution