Posting here after having searched for an answer myself, and this may help others (who google or search engine) as I have now solved the issue in my opinion. I acknowledge the various extensive conversations about what is and isn’t safe or suitable, and it’s not my place to make a recommendation. I am just stating the solution I chose to use.

In October 2025, I purchased and installed a:

Maxi-Cosi Tanza i-Size, High Back Booster Seat, 3.5–12 years, 100–150cm, Compact Foldable Car Seat, 10 Headrest Positions, G-CELL Side Impact Protection, Padded & Elevated Kids Car Seat, Full Black

This fits snugly in the carbon bucket. I have chosen to raise the electric seat to its highest position, and my 5-year-old can see well and sit comfortably. The seat belt loops through the non-ISOFIX tethers neatly (again in my opinion).

This is a niche car and a niche question about it, but I spent a lot of time searching and had to return two other car seats to get this solution. If this saves one person the hassle, then it’s worth the post.

Once again, I am not validating the legality of this solution but I am pleased and comfortable with the outcome.

Thanks to all who post useful information for others to benefit from as I so often do. Cheers Ross