I tried almost everything I could find on the App Store. There was something I didn't like about Spark, but I can't remember what. What I mostly wanted that Outlook had all of were the following:
1. Dark
2. Good swiping customization
3. An easy way to undo a delete
4. No subbing (I could live with a modest one time charge)
I know I'm late to reply to you; apologies. Regarding your points:
- Spark, as of Nov 2019, does offer Dark Mode support.
- Spark does offer swiping, although I'm not sure about customization. I'm not sure that Mail does, either(?) Also not sure in what capacity/use case you'd want swiping beyond what's available.
- Spark has a fairly evident "Undo" button when you've deleted an email, it just takes getting used to.
- Spark is free.