I was using Spark for the past few years because it was the closest thing I could find to Inbox that supported non-Gmail emails. A few months ago they put out a major update (v3) that completely borked it for me. It constantly crashes on both iPadOS and Android, so I've moved onto K-9 Mail and am waiting for Thunderbird to come out on iPadOS.

I didn't use K-9 for the longest time because I had a poor experience with it the first time I tried it many, many years ago. Between then and now it looks like they refreshed to Material Design and have significantly improved the UX. I'm happy with it.