Start base with gradual descent (3-400'/min). Flap 30.
Not necessarily so - especially at night where ground features may not be discernible and it is impossible to see if you are maintaining safe terrain clearance of 400 ft (Cat C). Once you deliberately elect to descend below the published circling MDA on downwind or base, you are entirely responsible for your own terrain clearance. Only a courageous pilot would risk descending on base until you are established within the approach splay. In some countries a PAPI or VASIS may be seen on base leg but obstacle clearance not guaranteed until within plus or minus 15 degrees either side of extended runway centre-line. Short answer: Don't descend until on final.
With regard to timing of the downwind leg. The timing policy was an old one from countless years back when bad weather low level circuits sometime encountered IMC during parts of the downwind leg. In those cases timing was the method used depending on height and it was acceptable to go IMC but for short periods only. Hence timing. In those days a circling MDA hadn't been invented and it was a case of preflight study of relevant aerodrome charts and familiarity with the terrain.
With the advent of DME or availibility of a reliable distance aid, there is nothing to stop you from extending downwind to keep within the limits of 4.2 nm circling MDA (ICAO). After all that is why the distance limit is published as part of aircraft categories. It is another story altogether if you are unable to keep the aerodrome in sight because of reduced visibility. A missed approach would have to be conducted. But if visibility is not restricted, then use the extra distance downwind to avoid the problem of being too high on base and final for a stabilised approach.
With a relatively high circling MDA there is a myth that at night you can descend below the circling MDA (potentially dangerous unless you know exactly the position of the critical obstacle that dictates the MDA) at any time downwind or base in order to suit the profile of your aircraft type. The designers of the published circling MDA couldn't care less about your profile descent problem. It then becomes your problem and the vultures await your custom if you choose to drop below the published MDA at night until safely ensconced on final.