I understand!
I just made a similar decision. It's tough.
My nearly 5 kids are placed where they are based on various factors.
So, my 3-year-old is RF in a Radian. In May baby#5 arrives. 2 months ago when I was very newly pregnant, I installed a FF Radian for my son to use once baby arrives (he'll be 3 months shy of age 4 and around 40 + lbs. and he's tall). The thing is...I'm guessing. Would he still fit RF at that time? Maybe. I even had to guess (I hope I'm right) to take Safe-Stop off.
The install was SO difficult that I almost couldn't do it. My hands were JUST small enough. I was up against the belt path and the plastic of the car, hanging upside down as I did this. These are two Radians side by side with the 40 part of the seat folded down, third row. I don't even know that two RF would puzzle like this and didn't try.
I know I couldn't do this any more pregnant easily, if at all. And I don't know what kind of recovery I'll have. Nobody (not even the carseat checks) have heard of Radians around here, so if baby arrived (and I was too sick or sore to re-install), and my almost 4-year-old didn't fit RF anymore...what would I do?
I'd probably pay a tech who knows Radians to switch it, but even so, the install was so awkward. Terrible. But it's done. He'll FF when he's almost 4. I probably overthink this too! Right now he's RF and I have to lean on my belly to buckle while bending into the hatch. This, obviously, will get difficult. I'll have to not lean on the belly somehow.
Note, though, that usually by age 4 or even earlier my kids could buckle alone in the Radian, so that's always an option!