@abudef I've been there, here is my vision on the topic.
in terms of RETENTION of the schedule, if you put 21, not using the FULL BACKUP INTERVAL in the job, you will get : one full and 20 subsequent deltas. on the 22nd point, a delta will be done and the oldest delta (the one just next to the full) will be merged in the full. its like a forever incremental.
Put 7 in FULL BACKUP INTERVAL, and every 7 days (if it runs once every day), it will make a full.
other way to do it : as you told, two schedules, with forced full on sundays.
difference between the two ? you really choose wich day the full occurs.
in the first way, the full is "7 days after the chain begins"
do not mix these two way of configuring, you would have unnecessary fulls in your chain.
in terms of LTR, @florent and @bastien-nollet recommended me this :
if you use LTR (lets says 14Daily, 4Weekly, 2Monthly), keep the SCHEDULE retention to 1
the LTR parameters will keep the retention points as intended
try not to mix with FULL BACKUP INTERVAL
LTR is still early, because i would like the WEEKLY/MONTHLY/ANNUALY to always be fulls or specific merged fulls, this is not the case...
it is still not clear, as you can't choose it, which day is the WEEKLY (sunday ? monday ?) the MONTHLY (1st ? last day of month ?)
trial & error mode activated.