Hi sorry, I was not on MN for the last few days.
I am not sure about using an Axkid wedge with a Britax MWP. I suspect you'd get a useful response if you contact Britax directly, though, or I've had some useful/practical/non-rabid advice from contacting carseat.se before - they absolutely love the max way plus, and would probably support you continuing to use it. I used FB messenger, but I am sure they also respond to email or there might be a contact form on their website. I think they'd probably be able to advise on suitability of taping up the wedge you have, as well.
The other places I'd look for advice might be whichever retailer you originally bought the MWP from or maybe the In Car Safety Centre due to the medical needs of your LO. Otherwise, the FB groups The A-Z of Rear Facing and Car Seat Advice UK seem to have less misinformation but also hardly ever have posting open. But TBH the way that FB groups attract the piling-on puts me off ever asking publically on there, even in those two groups. You could also try messaging privately to either Margaret from Rear Facing Toddlers, or Alan from Little Peas. I have seen both of them give reasonable, personalised advice before. Although they may not accept private messages from non-customers, I don't know. Margaret is on the A-Z page and Alan is on the UK one. The other reason to look at FB groups would be to ask whether anybody would be willing to sell or post you a no-longer-needed Britax wedge.
I'm afraid I'm not sure how big the newer Britax seats are to compare directly. I think the angle is probably the issue, they don't seem to adjust as much as the older Britax seats. Measuring from the rear seat headrest is tricky, because the angle of these varies massively and even the height of them would affect the angle as well. Remember that if the dimensions given are the car seat sat flat on the floor, it will be tilted back slightly in a car, as most rear vehicle seats do tilt backwards between 5-15 degrees. There are flat ones but they are much less common, and mostly in vans/people carriers/minibuses.
I will see if I can find any comparison photos of the size profiles of the newer seats. These used to be the bread and butter of those groups/pages and I miss them!
FWIW, if you still have a TWE, you're confident bracing it, and it's still in good condition, it is also still perfectly legal to continue using it, and personally I would be OK with this, especially if you can fit it in the middle - the side impact protection isn't great (but even RF itself is a protective factor for side impacts.) This Swedish study says 10 years is a reasonable lifespan for car seats, but also that longer is possible. Britax have always used high quality materials and I would assume that their build quality in general lends confidence to a longer lifespan. Anecdotally (and third hand) I saw someone in one of the groups say that they had visited Britax's German factory and at that factory, there was a (Britax-) Römer King seat (their flagship from ~2003) which had been displayed on the roof outside the factory, exposed to all weathers, for around 20 years. As an experiment, they took it down and crash tested it alongside a newly-made King seat and it only performed slightly worse. This study suggests similar results, although points out that newer seats often have better safety features, which is definitely applicable to the TWE.