Personally I don't think you need the extra harness capacity of the i-Irvana Max so I would vote for Evolvafix over that seat for the 3yo. The other good one is Maxi Cosi Titan (various versions) or Onyx, or Silver Cross Balance, either the standard or Essentials version which is cheaper. The Maxi Cosi and Britax seats are all around the same price, although the Silver Cross Essential seat is on amazon for £120 which is a good offer.
You could also look at Cybex Pallas B3 which is also £120 and turns into an excellent high back booster but is an impact shield seat, which you might be less keen on (safety of the shield is roughly equivalent to forward facing in a 5 point harness). Halfords actually have a clearance deal on this and the Pallas G for about £70 - I can't tell if this is some kind of mistake but that's an excellent price if you're considering an impact shield seat.
All of these need isofix and top tether. Show the top tether to the grandparents as it must be used every time. They are easy to use but are sometimes missed/forgotten.
The 10mo should be OK for a while yet in the Cloud. That might be worth keeping for at least one of the cars to save buying three seats? Once it's outgrown, you'll need a third seat but it's something for now anyway. You might be able to get another 8+ months out of it and move the older child to a high back booster in that last car.
It might be worth considering one of the folding seats which are on the market for one of the cars, perhaps the least used one - as they are handy for travelling, with 2 DC you may get that use out of it, and saves you buying a separate seat for that purpose at another time. However, I'm really not sure what the safety performance is like on these. The seatbelt fitted ones haven't historically got very good scores on ADAC (Which?) testing, but you can now buy a few which fit using isofix and top tether, which ought to be a safer bet. Whether they really are as good as a solid frame seat, I have no idea, but they will have passed the legally required crash tests, at least. The only thing I found of note is that they all include an alarm which sounds if the top tether is not fastened (completely useless in practice, as you have to turn it off to store the seat) and I read in the legal standard that it's only if the seat fails a rudimentary crash test without the top tether, that these alarms are needed.
For about £130 you've got Maxi Cosi Nomad XL or Cosatto On the Go. Or there is a cheaper one by Kinderkraft (Fix2Go) which is £79 and seems to only be sold on amazon.
In terms of cheaper seats which are still solid, permanent seats rather than folding, Graco Eldura is the cheapest I would use, it's only £50 so a very small outlay, but I don't love it in terms of the fitting mechanism, it is seatbelt fitted and prone to getting forward anchorage in some cars. It also has to have the belt tight enough which might be a problem if it's getting removed and refitted regularly. However it does meet the newer safety standard which is a plus, and the fitting is very simple. It's similar to the old Joie Elevate. Or Joie Elevate R129 is on offer for £55 at Halfords.
Joie Stages is another option which might work well, this can be a rear facing seat so the baby could use it in RF mode to allow big sibling to stay in an isofix seat until a little bit older, which is beneficial when forward facing. Then when older child is ready for a high backed booster, the Joie Stages transforms into one up to 125cm (approx age 7.5 from current centile). It's about £85.
If you were wanting something fitted with isofix and top tether there is Cozy n Safe Hudson which is about £90 but meets the older safety standard, or Graco Energi which is £99 and meets the newer safety standard.
The main problem with cheaper seats of this category is that the belt fit in the high back booster mode in a lot of them is poor and/or the protection in the harnessed mode is minimal. But this should be a good amount of options to look through and decide, anyway.