Hi OP,
Congratulations. I found Facebook marketplace brilliant for getting things quickly (and cheaply)!
Things I loved having (beyond Muslims and babygrows) were:
A next2me/Snuzpod or crib beside the bed
A close caboo/wrap style or ring sling
A flask for you (to have a hot cuppa)
Somewhere/something for baba to be put down in downstairs/when you are in the shower (so a playmat or bouncer or both)
Changing mats (one upstairs and one downstairs, with nappies, wipes etc)
A Lamaze butterfly/pram toy (helped on car journeys etc.)
I breast fed, and did later buy a pump/bottles but was glad I didn’t get one in advance as I would have bought something completely different. However, the Hakka is cheap and handy (silicone hand pump that collects milk letdown).
Reusable breast pads are on my “must buy” list for the best baba (but that’s because the disposable ones left me with an ichy rash that I don’t fancy having again!)
My LO is now 11 months and on formula since I went back to work. We never got a perfect prep, just use a flask with hot water, and it works fine! So again, most of those “handy to have” bits can be bought when and if needed.
Pram wise. Friends love the bugaboo bee for city, but it struggles with off path walks. I believe the cityminijogger is good too. (Despite being city based Ive a beast of an outdoors pram so probably not your best guide, but suits me as I love long walks).
Presume you’ll need a car seat too? I found having a car mirror helpful (one for babies headrest so I could see them).
Also: think about your hospital bag. There are a few great threads here recently on what to pack
Im not sure if your six weeks brings you to before Christmas, but if it does and your other kids are little, perhaps a small gift from Santa/you?