Games start on 7 Aug with the GF being played on the long week end of the 3rd October.
SANFL press release
https://sanfl.com.au/league/news/sanfl-releases-updated-fixtures-for-2021-season/
'“For many years, the October long weekend was a really successful tradition for the SANFL Grand Final so both the League and Clubs agreed this was the most favourable date to conclude what has been another unprecedented season due to the Covid pandemic,” Mr Chandler said.'
Since the SANFL believes having the GF on the long weekend is so successful, they could consider leaving it there.
It worked well with a 21-game season, but with an 18-game season the season would have to be started very late April or have lots of byes/split-rounds to drag it out to late October.
21 game season with 8 teams would work.