It's tricky because he is on higher centiles. That's the trouble I think.
Out of Bold R or i-Irvana Max, I would probably opt for the i-Irvana Max unless the price difference is significant. (it has to be the Max, the older i-Irvana doesn't have the 125cm/25kg extended harness limit.) Simply because of the newer regulation. I know many many posters have been happy over the years with Joie Bold, though so it would also be a good choice if you are using the TT/iso.
If you want to persevere with the front seat and don't have isofix there, then you'll need a belt fitted seat, so one of the three I mentioned before (Joie Elevate R129, Graco Eldura R129, Maxi Cosi Beryl). None are ideal because the weight limit on the first two is 21.5kg, which it looks like he might get to around 3.5 years if he stays on the same line (which he might not, but still), and the Beryl has a lower-ish top harness height, so any of these three won't last you as long as the Bold or Irvana Max. The Beryl is also expensive, though IMO it's one of the better extended harness seats as long as shoulder height is not an issue. I can't remember the exact height of it now, but from memory it was identical to the Joie i-Spin 360 so you could possibly do a sneaky try of one in a shop with him if you can't find a Beryl to view in person.
Our 2018 (I think) Peugeot 3008 had isofix and top tether in the front. I know the current latest model doesn't have it - I don't know whether the one in between did. I think ours was the first after they switched to the SUV style. We now have the 5008, which has isofix in the middle back seat although I think the view to the front is not good, and I tend to feel sick if I sit in the back of it for too long. Peugeot and Citroen are both PSA Motors so have some manufacturing similarities.
DS2 will be able to go into the Joie Every Stage you already have, used rear facing. So don't worry too much about this. That at least can be postponed for a couple of years, and you can cross that bridge when he gets close to 18kg! There is usually an ERF seat around the £200 mark, so it wouldn't hurt to try and put £10 aside a month as a sort of car seat replacement fund and if you end up not needing it then you can spend it on something else.