In search of a safe and inexpensive solvent that can dissolve depolymerized polypropylene waste plastic
Short answer: Not really, unless you can do the cleaning still at high temperature. Solid hydrocarbons tend to only become soluble rather close to their melting point.

The only reasonable solvent is petrol ether, i.e. a mixture of alkanes. Polar solvents obviously don't work, chlorinated solvents are ecologically problematic and only have the advantage of an increased boiling point. Toluene or xylene might be alternatives.

So basically what you should do is to switch your feedstock from polyolefines to liquid alkane (petrol, basically) before shutdown. I would however be amazed if your process engineers don't do exactly that anyway. Maybe they could do that for a little longer, or maybe your process should be run (at least from time to time) at a slightly higher temperature so solidified (i.e. crystallised) residues in the reactor can melt and be washed out.

And whatever remains after shutdown and cooling, you will have to scrape off by hand, sorry.

(A sloppily designed or maintained reactor with bad flow profile, insufficient/broken insulation/heating devices of course might or might not be fixable.)