I’ve struggled with this as well. My office computer is windows but at home I use an iPad and a MacBook Air. I’m pretty much stuck with Outlook on both my MacBook and office computers and saving into subfiles in outlook is tedious. What I’m trying now is dragging all my emails into a folder for a particular matter and then just deleting those same emails from outlook. I used to convert to PDF but its too time intensive. The folders I transfer emails to are on my office network and are backed up.
The iOS app allows you to put the pdf button on the menu, so while it is manual it shows up beside reply/forward etc and then jus click DTTG to import it to DEVONthink. Faster than copying the email with iOS mail app in my opinion.
After trying things out and looking at the beta ios16 mail, Spark mail is still my choice, and im not sure that I want to have a second instance of mail.app on my iOS/Mac just to import EML. The see email->pdf button->DTTG is quick on iOS. Me waiting to get to Mac or opening mail.app just for eml probably won’t happen.
I am a well known DEVONthink 3 fan. It is a heavyweight app., it is also a very lightweight app., if I can put it that way: It is not expensive and worth its price for what you are doing alone. You would, I guarantee find further uses for it if you got it.
As it happens I use HoudahSpot to search it as well as its own search functions. I rely on search now rather than filing systems so to speak. HoudahSpot, provided the native Mail app is actually open, will search Mail well in my experience.