Here's my opinion: Cloud is absolutely right that Pence or Ryan or Hatch or anyone else would sign the tax cuts just as quickly. The GOP establishment knows that, but they're afraid of Trump's voters. They've spent four decades using hate radio and Fake News to cultivate this mob of racist, heavily-armed religious kooks who are convinced that the dam librals are out to steal their guns and Bables and make their sons gay marry Mexican immigrants then let Muslins kill them all. The human dumpster fire has them convinced that he's their Messiah; if Dump is removed from power and the GOP establishment looks culpable in any way this mob--or, as you can also call it, the Republican base--will be livid. The GOP establishment--the economic royalists who actually don't give a flying fuck about gay marriage or abortion or Jesus returning and just want their tax cuts, dammit--need those votes to keep and stay in power.
You forgot to follow the money. Yes Trump is the king of free publicity, and yes everything you said is pretty much true, and yes, it's only the midterms coming up, but these tax cuts stink to high heaven, and even half the Republicans in the Legislative branch know it, but they're going to pass it because they'll be wiped out in the midterms if they don't, and it's not about appeasing (or even energising) the base, or having a win under their belt, you just have to look at the boondoggles they're putting in, the people they're hurting, and the only people who are going to benefit from this fiasco. It is about the Republican donors. There aren't millions of them giving $27 each. It's the super rich giving hundreds of millions, and if the Republicans don't give them the world's most expensive blowjob, they're going to lose a LOT of funding for midterms and 2020. No ads to feed FOX or Breitbart or Drudge. No ground support. No stickers or posters or fake news (made in America). No insane 'think'tanks. They're trying to appease the asshole portion of the 0.01%, no one else.