Thanks for these points.

Re #1 my understanding is that dropping to 4GB would bring the monthly cost down to maybe $20 or thereabouts. But we will need more than one community node. Maybe more like a dozen as we scale up.
The bigger cost will be the tech time to run them.

Re #3 I believe the problem with using standard SP delegation is scalability. Say you need 15SP (although I have heard mention of 30SP) to get going, just a thousand new accounts a week will require some account being willing to delegate 15,000SP per week for X weeks (until released back). And we need more than 1000 new account a week to grow well....

Re #4 if @oracle-d can handle it they will be a good starting point - but again scalability will be the issue. Particularly if using SP delegation instead of RC delegation to support the new accounts.

And I would say @oracle-d currently have a very underrated witness position (#133). They really should be top 50 at least now...