I don't remember which Mothers albums I'd heard before 'Weasels" but it definitely seemed pretty jarringly experimental at the time. But I quickly came to appreciate the less accessible tracks like "Get Any Onya", "Gas Mask" and "Eric Dolphy Memorial BBQ" and how Zappa brilliantly juxtaposes them with stuff like "My Guitar", "Oh No" and "From My Heart To You". Somewhere in the middle are "Orange County" which you mentioned but I'm surprised you weren't able to at least enjoy the beginnings of "Toad of the Short Forest" and "Dwarf Nebula"- both killer compositions IMO. And "Get A Little" might just be a short n' sleazy guitar solo but it's a damn good one.
Like Matt said, the album when combined with (the admittedly much more accessible) "Burnt Weeny Sandwich" can sorta be considered "Uncle Meat II" although I sorta prefer to think of The Mothers' first 3 albums as a sort of trilogy involving the homages to/parodies of the counterculture and "Meat", "Weeny" and "Weasels" as a 2nd trilogy more focused on surrealism and sophisticated instrumental compositions informed by classical music and jazz.
Re: "Barrett", I was similarly underwhelmed after hearing the first album but grew to appreciate the 2nd album's more relaxed and unified sound regardless of Syd's deteriorating mental state. Wright's organ and Gilmour's guitar provide a nice shimmering dreamlike quality and though I basically agree that "Maisie" is pretty stupid- though somewhat endearingly so, especially when Syd actually chuckles while reciting the lyrics, I like pretty much everything else. "Rats" is more subtlety ominous than scary IMO, just like companion piece "Wolfpack"- the main appeal of both is the imagery of the lyrics.
Don't have much use for "Marilyn Monroe Memorial Church" (like you say, kinda cool atmospherically but way too long) but "Tanz" is still one of Duel's best albums. Definitely one of the most bizarrely structured albums ever though and while I basically like the closing instrumentals, I agree that they're kinda anticlimactic after those epic first 2 tracks. Check out "Wolf City" if you haven't already- does a better job of balancing accessibility and experimentation.
Been awhile since I've listened to any Yo La Tengo but "Electr-o-Pura" is my least favorite of the 4 I've heard. I like the mellower approach compared to the previous album but the material is pretty weak. Fortunately this would mostly be remedied on the following album which is generally considered their best and I agree even if I'm not a huge fan of the group in general.