A few questions for collectors of M101-4/5 , E135, and E121 and their various backs

[Luke]

Hey Guys,

I'm finding myself interested in the above sets, mainly because the variety of backs makes them really interesting to me.

In doing research on these sets, I'm finding that cards I would expect to be marketed as part of a larger set, are usually listed separately (i.e. I would expect a D328 1917 Weil Baking card to be listed in a way that connects it to the E135 set, but it doesn't seem like that is the norm). As a T206 collector, it seems like all of the E135 (ish) cards should have the same catalog number to tie them all together. I actually think it might make these sets more popular if all cards under the same umbrella had the same catalog tag (but what do I know)

That's a bunch of rambling, but here are some questions:

Do collectors look at these cards like they are one larger set, and collect is by backs, or do you tend to look at all the different backs as separate issues? Is there anyone on here who has a E135 set with a variety of all the different backs?

What is the market like for the various backs? One of my favorite things about T206 is that they are really liquid. It's nice to be able to sell cards when I need to pay for something I just bought. For instance, if I have a 1917 D328 Weil Snodgrass and I decide to sell it, will it sit forever, or is there an active market for it?

Does the scarcity of these cards translate into value and demand? Using the same 1917 Weil example, most cards have POPS of just 1 or 2. To me that's exciting because they are so scarce. But, does anyone really care?

Who mainly collects a subset like 1917 Weil Baking? Are there collectors of the E135s who collect them with all backs, or is mainly player/team collectors and Louisiana collectors?

I'll probably think of more questions, but that'll do it for now. I did try the search function but didn't find anything. Hopefully people won't mind re-hashing an old topic.