So, ultimately it's a personal choice but I'll share our experiences...We did the CL FPs last April. The main reason we did it is because my kids had not been to WDW in 2 years and had not seen three of the nighttime shows so that was a priority. My understanding is that the reserved nighttime viewing is no longer included (I could be wrong about this, I haven't been following closely?), so that was a big part of the "value" for us. I can't find my notes right now to remind me of everything, but...
We really enjoyed what it allowed us to do, and I'm glad that we did it - and in certain circumstances we may do it again. Having said that, I don't necessarily feel like it made the trip feel more "relaxing" to us/me?
What I liked: Reserved seating for nighttime shows - all three of which some or all of us hadn't seen. Being able to avoid tier restrictions - one day, we did DHS in am and Epcot in evening and avoided all the tier restrictions and that was great! You'd have to look at what you want to do to see if that's worth it for you. For us it was pre-TSL, it may have been even more valuable post-TSL, although you mentioned that you don't want to do SDD.
What we didn't like: The "planned" part of our day was too long for us. Now, I might feel differently without kids, and you mentioned you're just with your sister. Usually we have one part of our day planned (AM EMH, dinner ADR or show, etc), then other parts are more relaxed. It was harder to do this with the 6 FPs, three of which I couldn't adjust on my own. I bolded your idea about doing the regular FPs in am, then the CL FPs in evening. I can see why this would be good but you might consider...I did different things depending on our plans, but if I didn't have the evening show reservations to anchor us to a park, I might make the regular FPs in the evening, so you can adjust them. I remember one day, I did a CL FP first thing in MK, then my other FPs so I could get more by early afternoon, then did 2 CL FPs where we were doing our evening show, but I'm not sure I would have liked that if I wasn't committed to the evening show? I'm not quite sure how I would organize if I wasn't anchored to a particular park/evening show?
I'm probably no help at all but I hope I'm making sense!? To summarize: I'm glad we did it, but not sure we'd do it again without the reserved seating for evening shows, except to help with crowds for SWGE or some other specific goal! Either way I'm sure you'll have a lovely time with your sister!