Hi Scott,
I might be able to help a bit, since I've seen you over on other threads and think some of our resort preferences may be similar! BWI/BC are among my favorite WDW resorts and I particularly love the BWI club lounge. But I also love Poly and have been staying there since childhood. So of course, I think split stays are perfect!
I also think your expectations are reasonable because you already know that Poly is different. Very laid back tropical atmosphere at Poly, which is fun and can be relaxing but definitely a different vibe from the peaceful relaxation we find at the Epcot area resorts. I love the new bedding and decor at Poly - that really helped bring an old favorite up to deluxe standards. And even though I've been staying at Poly since childhood and am very sentimental about it, I'm so excited about all the changes and think they'll have a really positive impact on the overall resort atmosphere.
Also a few notes about CL. Having stayed CL at BC, BWI, CR and Poly so far, we feel the Poly club lounge atmosphere is particularly different from the others. Just more casual, laid back, party feel. At first, I was surprised at how much more casual the Poly lounge was. But it really grew on me, and our boys love it so much there. It's also so convenient to the pools, amazing view, etc. - so many significant benefits that we'll continue to return. In our experience, CMs in the Poly lounge are friendly, but far busier than CMs in the other lounges. So they didn't have the time to engage and chat, like the CMs at BWI or CR. (I know you're familiar with the outstanding BWI CL CMs, and it's tough for anyone to meet that standard!)
I think Poly as part of a split stay will be great fun, particularly because you have a sense of the differences in atmosphere and vibe in advance. Happy to answer any other questions and compare resorts, if that would be helpful!