It’s not me this time! Not me! I didn’t start it again! 
I did the same calculation as you did, that Spark was the one sucking the least (I even devised an OF inbox perspective to specifically catch Spark deep links in notes for processing tasks). That is, if you use iOS and macOS on a roughly equal footing. If not, the balance shifts a lot when you have access to macOS’ scripting capabilities. At the moment, I find that Mail.app + Mailbutler is the most stable / powerful combination. (But fear not, it’s gonna change in in three months 
)
There definitely is a Spark 3.0 on the way (as stated by Readdle on Twitter). I really, really hope they revisit their task integration at that time. If they do, Spark is definitely going to be the clear winner.
I wholeheartedly agree with you: how is it that no email app does everything right? It’s not like it’s a very opinionated thing.