I'll let other more knowledgeable people comment on CGNAT.
Network wise, MR is not using Starhub's network. It still has its own independent network. What has happened after SH bought over MR is that MR is now using SH a lot more as an upstream provider. Meaning the route that your traffic takes to access the internet - much of the routing now goes via SH.
You might say in that case there's no advantage anymore to subscribing to MR? I can see three advantages:
1) MR still uses its own infrastructure. I don't know how true it is, but their contention ratio (number of subscribers sharing the same bandwidth) is supposedly lower than other ISPs.
2) Not all of the traffic goes via SH. I can still see some routes that are different (and better) than SH's, with lower number of hops and lower latency.
3) MR has free digital voice. SH must pay.
You may be better off with SH if you also use SH mobile or TV and get to enjoy the bundle discounts.
Technology wise CGNAT should not cause any problem if MR set it up right and have sufficient capacity, so for your needs it should not cause any problem. If they messed up, it will be the same no matter if you are on static or CGNAT. Anyway you can pay like 50 bucks to convert to static IP if it is really that bad.
MR is supposed to be managed separately from SH but I really doubt that that will last forever since it make no sense to do that when you can just pool the resource and probably provide a better solution.
If you really despise SH and buying MR only because it is using MR routes then go for something else, 2 years contracts is a long time, so things mightn happen.
