The only thing I'll add to this, is whoever is paying out the cash at the chop shop, is probably also the "guy" working there, thus it would make sense for you personally to not have to stick around for the chopping of said car, if you get the cash then walk away and thus the car begins getting chopped up.
So, proposition is, remove timer, increase cash payout to be better than the legal jobs. Not marginally better, but good enough to matter.
As for gangs taxing you, well... Get in good with the gangs mate or don't do it alone. That's just an RP thing, if cops wanna camp them, there's what... 4 chop shops? Can't all be guarded by police at all times, they got better things to do than sit there 24/7.