Hi and welcome! Since you got some good booster recommendations and also have the names of some nice boosters already in your first post, I also wanted to mention the option of a combination seat. Combination seats are harnessed seats that become booster seats later.
I personally am more comfortable with kids in the 4 year old range in a harness; I think many kids don't have the ability to sit properly in a booster at all times and could benefit from a harnessed seat as their main seat until the 5-6 year old range or beyond. :twocents: For a kiddo who is 53 pounds, I'd suggest researching the new Britax Frontier 90 or the older model Frontier 85 (which is still online) and also the Graco Nautilus/Argos, the Britax Pioneer, and the Evenflo SecureKids 300 & 400. The Frontier models have the tallest top slots and become a very tall boosters. Some combinations seats like the Frontiers become just highback boosters (but as backless booster are relatively inexpensive with some in the $13-25 range, and since you said that you are comfortable with that purchase later, I think a combination seat might be worth researching), and some become highback then backless boosters.
If you do decide to go the booster route, you may want to look into some taller boosters in addition to the boosters you mentioned in case your tall son carries a lot of height in his torso. The boosters that were recommended and that you noted in your original post I think are probably all good choices to research further, and some (like the Turbo and the Vivo) are often noted for fitting young booster riders well. However, there are also boosters like the Britax Parkway SG/SGL or the Diono Monterey & Cosco Pronto & Safety 1st Air Protect (these last three require head support from the vehicle even in highback booster mode) that have shoulder belt guides that go taller than some other boosters.
As you noted in your first post, at 53 pounds, he's outgrown his Roundabout 50 by weight. But with so many lurkers reading the posts on this board, I did want to make sure to point out in my response for any others reading the thread that a kiddo in a situation with an outgrown seat needs a new seat before riding in the vehicle that the outgrown seat was in again.
OP, good luck with your research! I hope you find a seat that works well for you and your son!