"Spyglass" and spying on her at all is not necessary to go on a date with her. Just get her Affection high enough (i.e. shovel a lot of dung). Tested it myself.I thought the v3-41 Update would make having a relationship with Beth not mandatory for the Main Quests; that the MC didn't need to help her a few times and go on a Date with her.
It is possible to ask Erevi for the Summon Spell even if you don't trigger "A Date with Beth"? Like, for example, never triggering the Spyglass quest?
The quests not related to the Main Quests are only the ones about the Pregnancy and Kidnapping?
Yes, after the date Beth's content is about pregnancy & the kidnapping, pretty much the same as the other Side girls. The goal was to develop Beth to where she could have kids (just Vix left for that from the existing girls). She might get more content later, many girls have had more added after kid production is enabled. But even then it's usually about sex/pregnancy as much as developing a girl's story/background.
TBH I'd like to see the Beth date downgraded from Main to Side without removing it. There's already several Side quests which are required for certain points on the Main questline. Recovering Edgar's stuff is the big one, but there's also getting Rosy's bakery running again, escorting Caleah to the swamps, etc. Or all those other Side quests could be upgraded to Main since they must be done for the Main questline...
But hey, my boy Gabriel got some development too!