Just because you don’t care about something doesn’t make it worthless or boring. If I didn’t think the OP cared I wouldn’t have pointed it out. Above all my point was constructive and took the other person’s interests into account, it was not a pedantic critique.
If you find the matter boring, I encourage you to move along. Prolonging it seems counterproductive.
(I don't actually know how often I send email from that address, so maybe I don't even need that, but just in case.)
I set it up a long time ago this way and I'm unsure of I had to somehow configure Gmail to be able to send from my other account. But I have the same also for @live.com and @zoho.com as I was trying different providers. This allows me to easily reply to forwarded emails with the old email addresses.
Edit: I think you got with the alias, didn't know this is what is called in Google https://support.google.com/mail/answer/22370?hl=en#zippy=%2C...
Feels overkill just to achieve the same outcome as we previously had for free, though.
I guess it applies to the free account too but they just don't bother to state it (yet).
Fortunately, they're all set up to forward to Gmail. Gmail never checks them itself.
Does e-mail forwarding not work in your case?
The GMail mobile app does support IMAP, but that is different from GMail as a service supporting it. The mobile app having IMAP support does nothing for people who use a web browser.