I don't know that much about it, sorry. My general feeling about combination seats is that they have to make a compromise somewhere. High back boosters should ideally be light and flexible, but a spin seat especially with a higher capacity on the harnessed mode needs to be quite strong and solid, so that's the only thing I'd be a bit unsure about. But all the ERF specialists who have actually seen it in person seem to love it and rave about it in a way they haven't about other all-stage seats e.g. the Britax Swivel or Joie Spin XL, and the belt fit does look good in booster mode, which is unusual in the combination seats.
Avionaut as a brand I do like. Their seats are really thoughtfully designed and I think that's a really good thing. Their safety reputation is also very good.
It is good that you get the spin up to 125cm. Not many seats on the market do that. I agree that the 21/22kg weight limit isn't likely to be a problem for most people especially if you don't plan to stay rear facing more than a year or so more.
In terms of saving money on not needing to buy a HBB later I think this is a red herring, you can get a good HBB for under £100 and the Britax 125cm seat is about £200 on offer, so you could get both seats for less than the Avionaut, though without the spin of course. OTOH it does seem a shame to go straight into a HBB if you could get a seat which can rear face for now. How far off 105cm is your LO? They tend to grow in arms and legs and in spurts so sometimes you can get longer out of a 105cm seat than you'd expect.
You might want to compare with Tinyseats 2 - it's more expensive but it does fold up for travel which might be a useful feature.
Personally I don't think there is much difference which rear facing seat, the difference is likely to be in the performance as a high back booster. As long as the seat is rear facing and meets some kind of safety standard, it's going to offer excellent safety. You don't need to spend out loads on a RF seat, the most basic cheapest one will get you the same safety because the safety benefit is in the fact it's RF. This one has not had an ADAC test yet.