Axkid say 8 years in use now: https://axkid.com/frequently-asked-questions/faqs/how-long-is-the-life-span-of-an-axkid-car-seat/
So if you want to, you can wait. If you're thinking about using the seat for another 2.5 years for example and that would take it to about 8.5 / 9 years old, I would be happy with that personally (as long as the seat is in good condition with all working parts) It's not as though the seat will become unsafe on the first day of the ninth year of use.
OTOH if she is only just 3, and/or it's already more than 7 years old and/or you're hoping to use it for another 3-4 years then it would make sense to look into a new one so you get the most use/value out of it.
Only i-size seats have the width restriction, not all R129 seats are i-size. It's a bit confusing because the first phase of R129 was just the i-size compatible seats, so a lot of people (including professionals) used the two terms interchangably so most people reasonably assume they are the same thing.
I-size seats are all isofix fitted and up to 105cm child height, so no use to you anyway.
You can still buy the Axkid Minikid 2 and Move fairly inexpensively - there are also newer/R129 versions of these seats, called Minikid 4 and Movekid. I believe these are still the most compact options on the market. They also have much better weight capacity and extend up higher in the headrest compared to the original Minikid. There is a good comparison video on youtube by Rear Facing Toddlers, if you wanted to see.
Britax have retired all their older ERF seats, but have a new one called Max Safe Pro, which is on a good offer at a lot of places currently. I'm not sure how compact it is but it's definitely worth looking at.
There is also Avionaut Sky - but I think this will be too short in the shell for your wishes. Besafe Stretch - but this is bulky. Klippan Century/Opti - but these are hard to get hold of and not that popular on the UK market.
Where did you buy your Axkid 6/7 years ago? Could you go back to that retailer and see what they advise? Axkid also have a stockist list on their website, so that is another option - see who is selling them in your local area and go and ask about the current models