Help w converting Frontier to Booster Mode

Hello,
We have been using the Frontier 85 SICT in Harness mode and I want to switch it to Booster mode. My son is 75 lbs and is 4'7 inches tall. Just want to make sure I am understanding the manual. Tried to call Britax but cannot get thru as usual. It says one can just place the car seat on the seat and then fasten the seat belt without using the latch connector. The seat is just loose in car unless the seat belt is actually fastened?? Or- one can use the latch connectors and the tether if you choose?
The purpose for not using the latch connectors is one can move the seat more easily? Is it not safer to just use the latch connectors?
Then my other question is how does one get the Harness off. I followed directions and unhooked it from the yoke on the back of the seat and pulled straps thru to the front. Now where do they go?? This does not seem clear in the manual. It would make it easier if they mentioned front, back bottom of seat so we would know what to do. LOL. Help!!! Another problem is friends are teasing him about using this seat since they all have just a small booster. Is this seat any safer than a little booster??
Also, I need another booster seat for my son in his dad's vehicle. Someone was so kind to give me a suggestion previously but I cannot find that post since it was months ago. He uses the Bumble Bum now in his dad's car but he says its very uncomfortable. Thanks and love this site!!!! You all have been so helpful for carseat issues.

Here's a video! Britax Frontier 85 Harness-2-Booster Seat: Converting from Harness to Booster Mode - YouTube

I always latch and tether ours. If you don't latch it, you have to buckle it in every ride in which it is empty, so it does not being a projectile.

How old is your child? At his size, a lot of kids are in backless boosters and that's a fine option. It doesn't surprise me he's finding the bubble bum uncomfortable at his size. I'd look at options with more leg support. He might even be large enough to fit the incognito, a great option for larger kids as they get close to 5 stepping, but still need a small booster for a while longer. My kids have a different build than your son, so I'm not sure which would fit him best, but you might want to look at a Cosco topside (this fit our friends of similar stats) or maybe the Diono Solana (from what I've read). I hope this is helpful!