I think between Slow Start and Defeatist,
Defeatist is the more promising route, because as mentioned, staying in for 5 turns is a big task without Toxapex-level bulk/movepool/typing. With Defeatist, there are several routes available to us.
One example is a
fast pivot. There are mons like Cinderace, Dragapult and Zeraora which operate around outspeeding most of the metagame, pivoting out, and rarely if ever taking damage. In this scenario, Defeatist is obviously still detrimental, but it can be circumvented through careful play. After activating Defeatist, this mon would be a much less effective pivot, so avoiding activating it is essential.
Another scenario is a
fat status-heavy mon. Raw damage isn't as important here. This mon could perhaps have reliable recovery to replenish its stats. This could be seen as somewhat circumventing the ability, but it can still be exploited by various means. Because this mon is forced to recover to have a reasonable damage output, setting up a substitute on it should be extremely easy.
A third potential role is a
wallbreaker with high stats (perhaps stats that would make it broken disregarding Defeatist, to make it usable) that hits extremely hard right out of the gate. Think Eruption Heatran: if it takes damage, it's weakened, but it can still blast holes in the opponent's team prior to that. This would be harder to balance, and I think access to reliable recovery would likely break the mon, but of course the ability would be exploitable. After taking enough damage to activate Defeatist, assuming no reliable recovery, this mon would likely be sack fodder.
Edit:
Out of all the bad abilities we have to choose from, Stall is the one I'm fascinated by the most. This is because I feel that it's one of the only abilities that give us multiple options on how we want to play it. Slow pivoting has been brought up a lot, so I'm going to talk about an underrated aspect of making a good Stall mon: negative priority moves.
Yes, I know, Teleport is a negative-priority move, but it's been explored enough. There are many more negative priority moves that could offer interesting niches for CAP 29 to take on. For instance, we could use moves such as Roar, Whirlwind, Circle Throw, and Dragon Tail to make it a phaser. Or we could have a user of an almost-guaranteed 120 BP Avalanche/Revenge. We could even make use of Counter and Mirror Coat of all things!
I really think that there's more to Stall than meets the eye, and that could make for a really fun CAP process.
I just want to mention that Avalanche, Revenge, Counter and Mirror Coat, as well as the phazing moves, all have heavily negative priority already. Because these moves are essentially guaranteed to move last anyway, they don't really have any particular interaction with our ability other than making the moves' downsides less relevant. Still, for these moves, Stall isn't really beneficial in any relevant way.