Potty training: stay the course or try something else?

Context: 3 and a half year old. Started prompting to use potty at 2 and then fully no diapers training at around 2 and a half, the same summer that his sister was born (this was not great timing but had to be done to comply with daycare). My recollection is he started out better than expected, got to the point where with prompting he would use the potty consistently and increasingly would warn us he needed to go.

But then he plateau'd: not moving towards independence, continual reliance on us prompting, accidents, etc. Then more recently he seems to have really regressed: he is having near daily accidents, not saying he needs to go, and often not even telling us he's had an accident (hard to read his feeling about it; sometimes he seems ashamed, other times indifferent).

One story I have in my head is that this regression is related to his sister's arrival. There is plenty of evidence on this front: he's jealous, wants to be mommied a lot more, has even asked to drink from a bottle once or twice. This makes me think: stay the course, see what happens.

But another side of me worries that we are not disciplined enough with him or conveying enough to him that having accidents is not good. We immediately change him when he has had an accident (e.g., play / whatever he was doing is interrupted) and I have recently started showering him to clean (reasonable to do on its own but also because he does not want to). But he just doesn't seem all that bothered. Should we up the "consequences" for having accidents? I have heard of some people making them clean up their mess, like wash their undies with their hands. I have some concerns about that, especially that he might decide "actually poop is fine".

We have also tried going in the "other direction" of zero prompting to use the potty, letting him get wet/dirty, etc. But never felt like that was progressing towards anything.

We are really lost and starting to feel like his daycare peers are starting to leave him behind on this front. Any input would be greatly appreciated.