Hey it sounds like you're both doing really well
Completely normal to have some variation day to day. I'm not sure how it works with preemies, but I expect similar to full term LOs in that the classic peak of colicky crying/fussiness is around 6-7 weeks, so you may just be in the middle of that, do you think? Or is she happy in the evenings, just not sleepy? Sorry, you probably said but I just can't find it right now
That BT settling is great
Many mums will just work on BT settling first as that sets you all up for a better night, and work on naps later. I just worked on one nap a day with shh pat etc with DD, and ended up far more relaxed about it that with DS, where I struggled through trying to be consistent with sleep training but just ended up with a fussy OT baby
The other thing that may contribute towards the evening restlessness could be OT accumulating through the day, it's a bit hard to see from the routine you posted, as it's an average of several days. If you could post, say, what she does today, then it might be easier to see if she's likely to have got OT by the evening and so struggle to settle. Mine often struggled with the evening CN, especially DS, as he short napped the whole day and just ended up so wired! But really, she is still very young and things will be changing all the time, don't forget there's usually a growth spurt around 6 weeks too so lots going on... Continue to be flexible about feed times, 2.5-3.5h during the day sounds good to me although sometimes it may be more like 2h especially in the evening, and we wouldn't usually recommend stretching to 3.5h just yet but if previous intervals have been shorter and she's short napped and then just got back off to sleep again or something, I'd let her go to 3.5h
Sorry rambly thoughts today, hopefully something in there will make sense
Just want to encourage you that you're doing really well, it's such an intense time especially with an older LO too, so hang on in there it will get easier eventually