[Klaus]

The Sony FE 20mm f1.8 will pair great with an Astro-converted 5D mkII. Oh, wait... ;-P

The logic is actually very simple.

For ultra-wide shots I can take the Sony 20mm/A7R II - because the landscape requires its natural colors thus I can go for an unconverted camera. After all, I have a few options here. ;-)

For nebulas I can take the converted 5D II with some telescope or cheapish tele. 
E.g. this guy here ( https://www.youtube.com/watch?v=6WuBKfo_KMo ) is using a Tamron 150-600mm - and he's doing astro workshops using such setups. Now I wouldn't choose a Tamron, of course ... ;-) ... but this provides a direction.

However, let's come back to my question.

Is astrophotography really worth it? Ultra-wide shots of the Milkyway - Ok - this is a no-brainer because it doesn't really require any extra (specialized) gear.
However, is shooting nebulas really a thing other than just an eternal hunt for evermore perfect shots burning more and more money in the quest.
The amount of "composition" involved here is rather limited after all.