I traveled alone frequently with infants. A couple times with an infant and a toddler, which requires a bit more planning.
For 3.5hr flight, I would probably do (and did) lap baby, but that's me. If you can swing business class, for me that was a better option than two seats.
I recommend a travel baby carrier, because it makes it easier to board after you gate check the carrier. I had a travel ErgoBaby. Looks like this is the current version:
https://ergobaby.com/the-away-baby-carrier
Unless the person picking you up has a carseat that you've shipped to them/approved, I would not rely on having someone else in charge of a carseat. Car services that come with a carseat might not have something appropriate for your child. We always checked our carseat.
If you are planning now, your baby is much younger than 6 mos. Things like scheduling feeding etc will feel much less overwhelming at that age than for a younger newborn. So I would wait to worry too much about that.