I've been following this from the start and it sound like an interesting and large project. By large I'm comparing in relevance to your property size. Perhaps it is just me but your plans seem a bit convoluted and lacking focus.
Could you clarify what your main goals are and their priority? Culverts and retaining walls are secondary to the main goals. Are you looking to: make a flat driveway to the future shop? Get rid of the swale 'cause you don't like the dip in the side yard? Make a flat backyard? Something else?
You nailed pretty much all of it.

It all starts with creating a "dip-less" driveway from the road to where the garage would be (shed in the pics). Based on that - I need fill dirt.
So looking around, I came up with the idea of doing some leveling of the backyard - it is less than ideal - not horrible though. Granted, the amount of dirt coming from the backyard would exceed the amount I need to just do a driveway. So it expanded into "filling the dip" across the property from the tree line. The added benefit of this is a more level surface ( not perfect but better) - which would be great for soccer, football, etc. I have identical 5yr old twin boys and a 20 month old daughter.
Yes - the project grew considerably, but I am looking at the property as a whole - doing things in stages means trucking dirt in and out, more time as a muddy mess, more $$ for getting heavy equipment multiple times.
Convoluted and overkill? possibly - won't argue that. However, if I can do it once and we (including the wife) are happy, I think we could save $$ over the long term (yes more $$ at first)
Sort of like the time and $$ I spent on upgrading my stock Jeep CJ7 axles, then upgrading to Dana 44s, and now I have one tons (Dana 60s). Should have bit the bullet and gone to them right out of the gate - would have saved time and $$.