You don't necessarily need to swap to a backless booster ever. You can keep them in a high back right up until they no longer need one if you want to and they fit safely. It is usually more comfortable, it is safer, and it offers support e.g. if you ever do long journeys and they fall asleep.
Sometimes you get to a point where the headrest won't go up any higher and the shoulders won't fit any more, but they still need the boost to keep the belt off their tummy, that is a good situation for a backless booster. (If they are under about 8 or 135cm when this happens, it is worth shopping for a longer-lasting high back booster.)
Or for keeping for spares/lifts/travel, it's less to cart around than a full sized high back.
Or if you have a shorter than average child who is very worried that their friends will tease them about being in a "baby" seat it can be a good compromise. But I wouldn't pre-empt this. It's an option I'd consider if my child was very distressed.
Front seat - consider where the airbag would inflate. If his head is at adult chest height then he's at risk from the airbag, especially if he has his head bent over looking at a screen. Some countries it's illegal before age 12. It's legal in the UK but bear the airbag info in mind.
The 135-150cm is a bit of a grey area where they might fit some cars without a booster and some cars they still need one.
I would also not really go by what the majority of other people are doing on this one - I think we're in a bit of a blind spot currently, where a lot of parents of older primary DC are unaware that the law states to use a booster seat up to 135cm minimum, and they stop using any seat at all at about 6 or 7. My theory is that adults born between about the mid-80s to mid-90s but who didn't have DC until 2010+ are familiar enough with car seats from their own childhood memories to take it for granted that they are needed for younger children, but they were already too old to need a car seat, and too young to have had kids when the rules changed 21 years ago, so they aren't aware of the law saying they are now needed up to 135cm/12 years old.
It will disappear shortly once the majority of older-primary parents start to be 90s-or-later babies because they will have grown up with the new law, with their parents being made aware of it by the publicity campaigns at the time.