Is keeping a booster buckled and have child climb in and out safe?
It is safe, so long as you make sure the belt is positioned properly after he climbs in (slack removed, lap belt down low on his hips, etc). It won't be super convenient, though, which could get old fast if this is the situation in your main car. What kind of car do you have? Since you've chosen two very narrow seats for your younger children, you might find you have room to buckle and unbuckle if you choose a narrow booster too!
We have an '04 corolla and we are able to get a FF Radian, RF Radian, and a Harmony LiteRider booster. Actually, now we have two FF Radians and the Harmony. We had a turbobooster and it was a knuckle bruiser to buckle. Our 7 year old is in the harmony and he can buckle himself in (although he has developed a definite 'technique' to doing so involving wiggling around.
Yeah, our kiddo just turned 8 and he is still in the high-back portion of the Nautilus in our everyday car (an Odysee). I would prefer a high back if they are going to be sleeping still too.