Hi OP, as she's on the bigger side I agree that a spinning seat is likely to be less useful to you, unless you think you'll have another child within about 2-3 years - then it can work fine, as you just purchase a larger rear facing seat for the older child when they outgrow the spin seat (about 2.5-3 years for children on high centiles) and the new baby can move into the spin seat.
The Minikid (2), Move and Maxway Plus are not suitable at 5 months. They are more suitable for young toddlers (9-10m+). You would need a larger infant carrier or cheap 18kg seat (e.g. Joie Tilt) to fill the gap until you can use these.
If you're wanting one that you can use immediately and rear faces the longest these are what I'd look at. There are a few more options than there used to be, luckily - it used to be a bit of a "dead zone" between smaller infant carriers and the larger rear facing seats from 9/10m+ but this isn't the case any more :) Unfortunately they are on the pricier side, but compared to buying an 18kg seat + £200 25kg rear facing seat it works out about the same.
Avionaut Sky - cheapest option (£380) , suitable from birth, lasts until approx 120cm
Axkid Minikid 3
Besafe Stretch
Klippan Opti
= All around £450, from 3 months, last vv long (32/36kg or 125cm)
Joie i-Prodigi
Axkid One
= Suitable from 3 months, isofix fitted (rare) but expensive - £5/600
Besafe Stretch is IMO the best but have a look at all of them, there aren't too many.