I'm going to update Umbra to 5e and use that. But, I'm going to update it a bit, so that my PCs are enemies of the antagonist from the start in Sigil. Like dajr, my PCs will spend a lot more time in Sigil itself, that's the whole point of Sigil and Beyond, after all (well, not the WHOLE point). Also, there will be a lot more effects of the glitch, and they won't be the only one trying to figure it out (and there may be other factions, maybe even Gods who think this will break Sigil) trying to keep the glitch going. My first campaign where the stake really are the world(s).
I'm OK with the story. I think it's fine.
What I will do is at the end of chapter 1 I'm going to work in a lot of shenanigans inside of Sigil. When I ran Dragon Heist I regretted not hanging out more in Waterdeep. I won't make that mistake again. There are a few things I'll nab to help me. Encounters Enhanced from dmsguild for one for a bunch of little encounters and hooks. I'll use Forge of Foes to create any potential combat antagonists. Also Shops Ins and Taverns on the DMSGuild.
Also I might extend chapter one with the mortuary.
I'm looking at Guildmasters Guide Ravnica too, see if I can steal some stuff from there.
Oh and the Train heist. In fact after chapter 13 the Train Heist will be the way they get back to fortunes wheeel. A particular fiends name is a portal key into Fortunes Wheel.
they might also stumble upon someones homestead within the outlands and have a chance at a game of cards.