I purge 2 weeks old media using these. Then I purge the largest rooms' history events using these. Then I compress the DB using this.

It looks like this:

export PGPASSWORD=$DB_PASS
export MYTOKEN="yourtokengoeshere"
export TIMESTAMP=$(date --date='2 weeks ago' '+%s%N' | cut -b1-13)

echo "DB size:"
psql --host $DB_HOST -U $DB_USER -d $DB_NAME -c "SELECT pg_size_pretty(pg_database_size('$DB_NAME'));"

echo "Purging remote media"
curl \
	-X POST \
	--header "Authorization: Bearer $MYTOKEN" \
	"http://localhost:8008/_synapse/admin/v1/purge_media_cache?before_ts=%24%7BTIMESTAMP%7D"

echo ''
echo 'Purging local media'
curl \
	-X POST \
	--header "Authorization: Bearer $MYTOKEN" \
	"http://localhost:8008/_synapse/admin/v1/media/delete?before_ts=%24%7BTIMESTAMP%7D"

echo ''
echo 'Purging room Arch Linux'
export ROOM='!usBJpHiVDuopesfvJo:archlinux.org'
curl \
	-X POST \
	--header "Authorization: Bearer $MYTOKEN" \
	--data-raw '{"purge_up_to_ts":'${TIMESTAMP}'}' \
	"http://localhost:8008/_synapse/admin/v1/purge_history/$%7BROOM%7D"

echo ''
echo 'Purging room Arch Offtopic'
export ROOM='!zGNeatjQRNTWLiTpMb:archlinux.org'
curl \
	-X POST \
	--header "Authorization: Bearer $MYTOKEN" \
	--data-raw '{"purge_up_to_ts":'${TIMESTAMP}'}' \
	"http://localhost:8008/_synapse/admin/v1/purge_history/$%7BROOM%7D"

echo ''
echo 'Compressing db'
/home/northernlights/scripts/synapse_auto_compressor -p postgresql://$DB_USER:$DB_PASS@$DB_HOST/$DB_NAME -c 500 -n 100

echo "DB size:"
psql --host $DB_HOST -U $DB_USER -d $DB_NAME -c "SELECT pg_size_pretty(pg_database_size('$DB_NAME'));"

unset PGPASSWORD

And periodically I run vacuum;