You can't get everything you want in one seat unfortunately.
The Sirona is fine, it has a good safety rating, it will allow rear facing up to 105cm and can be used forward facing as well. It is not the most compact seat - the spin seats generally aren't. And personally, I think it's overpriced for what it is. For example, this is very similar in quality, brand reputation and features, but it's much cheaper, has a higher weight limit and won't take your base out of service if you were to have another baby. https://www.boots.com/britax-romer-dualfix-m-plus-car-seat-space-black-10323552
Or if you wanted to go for an even more budget option, Joie 360 Spin GTi shares many of the same features and is cheaper again. I don't think the quality is quite as nice with the Joie as it is on the Cybex or Britax though.
But if you wanted the option to rear face even longer, the 105cm limit seats don't last as long as the 125cm rear facing seats. The 105cm seats also have limited leg room compared with the 125cm ones (which I tend to think of as "Swedish style"). The Axkid seats tend to be the most compact here. However, they don't offer the option to forward face.
After a 105cm / 125cm seat you would move up to a high back booster, yes. If you get to the top limit of a 105cm seat and your child is not yet old enough/sensible enough for a high back booster, you'd be looking at a 125cm seat to move them into, so if you think this is likely to be a problem later on it can pay to go for a 125cm limit seat straight away. Worth mentioning that you can still forward face after the 125cm seat (when you move to high back booster) and you wouldn't necessarily need to use it right to its highest capacity, but it will just get you past a hump if you aren't quite keen for your DC to go into a seatbelt at 106cm.
For a boy on 50th centile, 105cm is likely to be reached at approx 4.5 years, whereas 125cm is likely to be reached at approx 7.5 years. But growth spurts can also happen - these are just averages.
Currently there are no newer regulation (height based) forward facing 125cm seats on the market, only 105cm limit, though some of the 105cm limit seats do have more room in the shoulder height than others. It might be that over the next 3 years this will change, but I can't say. You can still get some with the old regulation which is weight based so 25kg limit harness, but there are only a couple of them around.