I used Spark for an extended time and was happy with that.
Then they started launching their "upgrades" crap with AI+ and everything else and I only got frustrated with their design and technology desicions.
What really pushed me over the edge was that they removed the "compose" button from their main UI in a update/pre-release. Reached out to their support since it surely must have been a bug, you can't have an e-mail client without a compose button I thought.
Nope, it was a deliberat desicions, wasn't needed. You are supposed to use keyboard shortcuts instead.
Got fed up and returned to Mail.app.
Don't like it but it's better than Spark and all of the other options out there.