I understand lawyers love hypotheticals! ;-)

Yes! And the only thing we love more than answering them is clarifying them!

So I'm going to assume two different scenarios - a stroke severe enough to incapacitate (or kill) Biden, and one that's not so severe that he has to stop being President but one where he withdraws from the race.

The first one is easy, under all the scenarios. Harris becomes President (immediately), and the Democratic party rallies around her new Presidency. There's still important work they need the President to do, and so they have to make even a year (or half-year) of her Presidency work. With that, none of the candidates-in-waiting-for-2028 and their donors (Newsome, Booker, Buttigieg, etc.) are likely to gamble on trying to wrest the nomination from the now-incumbent Harris in a weird cycle.

If Biden stays President, though, the different time frames result in different degrees of chaos. The worst is if he bails out next week. There's not enough time for anyone to get enough delegates to win the nomination. There is enough time for the well-established players to get on enough ballots for a non-trivial plurality of delegates, and argue at the convention that whoever gets the most delegates should be nominated. But because some candidates will try but fail to get on a lot of state ballots, there will be lawsuits and recriminations and arguments galore.

If Biden has a stroke partway through the primaries (say, late February) it's easier in many ways. No one will have delegates (the ballot deadlines close in early March). They'll just throw open the convention. It will be messy, but less messy than the first scenario.

And if he has a stroke in October, the Democrats will simply lose the election. Early voting will have started in some states, there's no process for replacing Biden at the top of the ticket, there's no constitutional rules for what happens if a dead person gets votes or electors, and without a candidate there's just no way the election day GOTV works.