Score the paint with coarse sandpaper so the stripper can pentrate some , but stay out/off of the plastic. Expose some edges with finer paper.
The Castrol & other detergents get under and lifts well... if it can get under. It's paint penetration depth seems limited. Overnight soak and a bit with a stiff toothbrush. Rinse a whole bunch and let it sit again overnight, re-wash and clean with a pre-paint solvent of choice.
What didn't you like? Mixing layers of paint types takes technique. Mixing brands is risk one, paint chemical type the next, and that can get messy.
Too wavey? Too deep? Too glossy?...
Tin, hi-rail, traditional postwar?
Some of the depth will get lost in a satin or flat top coat. Decal plans overide anything really.
Masking on the gloss for decal, and or feather sanding as best you can up to it, only on gloss soon to be flat.; is needed for the new coat to grip onto. (a haze with a scotch brite pad is fine. Glossed surfaces resist adhesion. This sanding will mostly magically disappear with any clear topcoat, and totally when you nail it. (have to rinse and wipe the clear sanding dust off well, that dust causes more haze and depth ripple than the sanding micro-scratches.