It’s commonplace for maintenance issues at non CANDU reactors to extend the “refelling” downtime. So no, CANDU can’t stay online for decades without maintenance and instead have similar downtime issues. The major benefit is when manufacturing plutonium due to shorter fuel cycles, but overall CANDU reactors are less cost effective which is why adoption fizzled out.
There are several situations:
* refuelling occurs
* maintenance and/or inspection needs to happen
* refurbishment may be desired
Some of these things take downtime, some do not, depending on the design of the reactor and other factors. A blanket statement of "refuelling forces a downtime" is incorrect in many situations. Depending on the maintenance (or inspections) needed, this may or may not need downtime. Refurbishment generally means tearing things apart, so usually means downtime, but if a site has multiple reactors they can be done one at a time, e.g.:
* https://news.ontario.ca/en/release/1006993/ontario-delivers-...
You said 27 days of planned outage is crazy uptime in the world of electricity production. Hydro routinely beats that by an order of magnitude.