Spark was pretty convenient until they changed to a subscription model, removed a bunch of features, and completely fucked over everyone who paid for a lifetime membership.
Idk if it’s available for iOS, but Fair Email is an amazing email client. It’s privacy focused, FOSS, and works well. Anything else will be inferior, either costing money, tracking you, uploading all of your emails to their server, or any combination of those things. You can try K9 email if it’s available for iOS and Fair Email isn’t. It’s a similar app to Fair, but hasn’t been updated in a long time on Android.