The recommendation is just to keep them RF as long as possible really, with the absolute minimum for FF these days being 15 months. It's more to differentiate between what it used to be a generation ago which was rear facing is only for the baby seat, whereas today most of the second stage "toddler" seats can rear face up to approx age 4, obviously size is dependent on the child similar to clothing sizes. (Many can also FF from 15 months - more people are turning at around 2-3 years these days) But I can see you are already up to date OP 
The Graco spin seat, which is a clone of the popular Joie Spin 360, doesn't have much leg room rear facing. You can use it up to 4 - many children will happily lay their legs over the sides or rest them on the back seat (this is perfectly safe) but we found we had to remove DC's shoes for comfort rear facing after about age 2.
Joie i-Pivot is not much more expensive and does have slightly better leg room. I think the Graco Slimfit is also OK for leg room, not fantastic but not awful. That one doesn't have a forward facing mode with 5 point harness at all, the only forward facing is the high back booster mode using adult seatbelt. Some people are opting to skip the forward facing with 5-point harness stage entirely, indeed this is where the "up to age 4" recommendation comes from, because 4 is the minimum recommended age for moving to a seat which uses the adult seatbelt (high back booster).
If you did want a seat which will guaranteed get you to age 4 RF and has better leg room, the Swedish type seats are worth looking at as they go up to 125cm which is around 7 years for an average sized child so they have plenty of leg room for younger children as they can accommodate a (tall, Swedish!) 7yo's long legs. However these have a much higher price tag, and you often have to either buy online or visit a specialist - the cheapest one is usually Britax Safeway M when it goes on offer (but it's still £200) or the easiest to use/most lightweight is Axkid Core, or Avionaut Stardust if you did want a spinning/isofix one. (Both of these are £349 though). If you look on Axkid's website they only make rear facing seats, so their stockists are often carriers of the other ERF seats as well. I don't tend to think of these as candidates for "spare" seats because of the price, but just to cover all bases!
I've attached a picture someone shared with me years ago and gave me permission to use although I've since blocked out the children's faces. The little girl closer to the camera (approx age 3-4) is in a seat similar to Graco Slimfit, and the boy further away (approx age 5-6) is in an Axkid seat. This shows how the leg room works in those different kinds of seats. Young children usually find it comfortable to sit with legs crossed. Lastly in the blue seat which is like the Graco Turn2Me is DS2 at age 2.5 and then at age 5! He was slightly over the weight limit then and it was not his usual seat but I just wanted to try him for a picture to see if he still fit, and he did.