I've worked in Deliverability for a long time now, across multiple sending platforms, and I go to industry events and know a lot of people in the space. One of the things I've learned from talking to colleagues at different companies is how opens and clicks are almost always lower after a sender switches ESPs. Not forever, and not lethally so, but regardless of which platform you used to use, and which one you use now, opens and clicks will be lower on the new platform compared to the old platform. Clients can be confused about it, assuming the new platform must be doing wrong, and deliverability people are invariably stumped when they first run into it, because there's no real handbook or guide to walk you through this.
So, when you run into this, keep in mind that you're not crazy. It's not just you. And don't just take my word for it. E-goi's Ruben Alvim explains this phenomenon very well right here. His article is from a few years ago, but still full of pretty good guidance. Leaving behind unengaged subscribers is not necessarily a "must" but if you can do it, it'll definitely help address this issue. And for sure, don't set yourself up to accidentally mail unsubs or addresses suppressed by the old ESP due to bouncing. If you transfer data properly, build up your reputation the right way via IP warming, and continue to send wanted mail, things will calm down and you'll be very likely to get back to those higher response rates after a time.