I know where you're coming from. I've experienced the shock of switching from a nimble startup to a large tech co with teams for UX research, web, mobile, backend, platform, infrastructure, observability. Took a good while to get used to it. Each project has had it's prototype phase where core stuff gets figured out and a proof-of-concept validated. The productionization and stream of shipping PRs to deliver that over the next N months can be a lot. Then a new project starts and things are new and interesting in new/unknown ways. Pair programming helps, it's like watching a movie you like where you know what happens with someone who hasn't seen it before.
One thing that would help here is if new initiatives are new delivered artifacts rather than being tied to the CI/CD process of the established product. I haven't worked at a place that does this, but have worked at a place that didn't to its great detriment--instability of core product from changes made by new and slow new product development hindered by being in same monolith. Whenever I see a posting about a bank or other saying that we're big but have a small startup-mentality team, I don't buy it. Would love to find out I'm wrong on that.
I can't say for sure if any of those many startups where I did really cool stuff still runs those products. Possibly only in a sort of maintenance mode. What I'm working on now has more chance of being the foundation for something that lasts a lot longer, so taking a very long-term view makes it worthwhile.
I still do look fondly on the scrappy startup style of working in a small tight platform/API team where you can have a super technical discussion using so few words because you already know where the thoughts are headed.