On v6 first: creating and editing backup jobs still happens in XO 5 for now, XO 6 shows the jobs but doesn't create them yet (https://docs.xen-orchestra.com/backups-and-dr/backups-in-xo6).

For the schedule, @abudef asked almost the same question earlier this year. @Pilow's answer there is clearer than anything I'd write: https://xcp-ng.org/forum/post/104108.
As I read it, you either use one daily schedule plus the "Full backup interval" setting (the full lands N runs after the chain starts), or two schedules on the same job with "Force full backup" ticked on the Sunday one.
That second way is how you get to pick the day, and he says not to mix the two. One thing to expect, from @Danp in another thread (https://xcp-ng.org/forum/post/91943) : the oldest backup is always a full and old deltas get merged into it, so for a while you'll see two fulls on the remote rather than weeks 1 and 2 being deleted. I don't think the scheduler can say "every other Sunday". I might be wrong there; someone who runs this kind of plan may know a trick.