Legally the seat is outgrown at 106cm, so technically it's a non-regulated use, but if he physically fits into it and isn't over the weight limit it's likely nonsensical to say it isn't safe. There are some caveats to this - e.g. if you had a very cheaply made seat, if it's a rear facing one fixed by seatbelt and at some point the balance of their weight tips above a level, if there are different limits for different modes, like some seats have an 87cm limit for rear facing but 105cm for forward facing - I'd listen to this. But in this case where the harness has a physical limit around this size anyway, it doesn't really make a lot of sense that if a child has more length in their legs it magically makes the seat less safe than it is for another child with more height in their body.
But also - 100-106cm is approximately a year's worth of growth. You don't need to worry about this yet, unless you're passing the seat on to a younger sibling. I would wait and see what weight and age he is when you get closer to the 105cm mark. It might be that he hovers at 100cm for ages.
There are harnessed seats up to 125cm, but they are currently all rear facing. There might be some harnessed FF seats up to 125cm in the future. Besafe has one on the continent but it's been delayed for the UK - also won't be any good for you, as it's one of those modular systems, similar to what you have, so will work out very expensive.
If you prefer forward facing, there are several old-regulation seats still on the market with decent space in the 5 point harness. I would look at Recaro Tian, as it performed very well in the ADAC test, it's a belt fitted one but it did as well as the isofix + top tether types. Of course being forward facing, it didn't do as well as the Besafe ones which can go both ways, but that is because they take an average of all modes the seat could possibly be installed for the test score, unless it fails one of them.
There are also several 25kg limit seats available under the old regulation (e.g. Joie Bold) however the newer versions do have the 105cm limit, so be careful what you buy.
If you get to the shoulder belt limit and are still unhappy with him being in a seatbelt, but are happy to ignore a height limit if he still fits into the seat, the Maxi Cosi Titan i-size has a generously sized harness (unfortunately, the non i-size one is very limited). The Silver Cross Balance and Britax Swivel do as well, though the Britax seat is only a booster up to 125cm rather than the usual 140/150cm limits.