I have never use CelcomDigi fibre before thus unable to comment much about their package or services.
I've been using Unifi (Home) and of course there will be downtime and luckily very rare. So far their services is an a OK to me.
As for Maxis Fibre, the router provided usually comes with a 4G SIM card thus you will have a continuous internet connection during any downtime of the fibre cable.
Any downtime with Maxis will be a kicking the ball around, thus the hassles but Business package seldom have this issues... occasionally lar...
The best package and services was TIME, their engineer is tip top skills and very experienced too.
Very rare there is any downtime, even if it happens the recovery was very fast like 1-2hrs only.
I can't comment on 5G because my area still haven't got any 5G coverage albeit seeing new 5G tower here and there.
If you feel that you are being tricked into signing up or extending/upgrade package, always go to the nearest office/branch and make an official complaint with them. Calling them is useless.
Threaten them with a MCMC complaint if it was not resolved, sometimes these callers are AGENTs thus they just want the commissions if you upgrade the packages.
Good luck on your adventures...