Another big challenge that I see here is how we avoid it being just another way to call for Steemit to do more stuff or give more upvotes.

I think that if we work out - preferably together with the Steemit team - a few criteria that are quasi indisputable, this problem can be avoided. There are actually some (few) users on the Steem who are willing to think about a positive future of the Steem even without a guaranteed sc01 vote.
This is not pure altruism: if the Steem goes a little more in an attractive direction, I personally have something from it too, because in the long run I would feel much more comfortable here. Will be a long process, of course...

I'm not familiar with Dropbox Paper, but I guess it's something like Google Docs?

Yes, that's about the same.

In addition to the people here, I'd also add that @pennsif is really good at keeping his ear to the ground and assimilating the thoughts of the Steem user community

Great. I would welcome his input. In teamwork... ;-)
And I think this teamwork is something we should just start, then we might see more clearly. It simply brings nothing, if now every "understanding thinker" knocks out a post and you have to pick up the important aspects again laboriously. A "central place" (as you address it later), to which one can refer " straying" users, is important.

I'd like to see a heavily moderated Steem community established for this purpose and a single "pinned post" as an index to track strategic areas of focus and the related "Steem improvement proposals" (with no other "pinned posts").

Fantastic! And probably the only way to clearly structure our work with the possibilities of the Steemit frontend!

(i) What to do about tasks that are too large/complex to be described in a post and supported by upvotes; (ii) How/When/if ever to link up to the SPS?

Hm. Shouldn't we just get started on the work before worrying about "compensation"? If the teamwork is effective, the ideas will come naturally. If it is without the desired effect, we will have to think about something else under the aspect of "trial and error" anyway.
Haha, that's how the Steem works.... ;-)