Unable to load all emails on iOS Mail app.

I recently made the move to iCloud mail from Gmail. I used Mac Mail to copy over all of my email messages to my iCloud account and then set up auto forwarding from Gmail so that iCloud will receive all new email. It was about 15,000 emails going back to 2009. Everything works perfectly fine on my Mac Studio and 2019 MacBook Pro. All emails appear correctly in iCloud Webmail as well. The issue is with the Mail app on both my iPhone 13 Pro and iPad Pro 3rd Gen.

My iPhone initially showed only a few emails. These emails were chronologically in order, but there were huge gaps of emails missing in between each message. Scrolling to the bottom of my emails would force Mail to load more messages, including filling in most of the gaps that were missing. So I literally spent a couple hours scrolling just to load up my inbox. When I got past 2016 emails, I began to run into some issues. Emails from 2015 and 2014 would start to load in, and I could see the flagged message count rising as they loaded in (I refer to the flagged count because iOS mail doesn’t display a count of the inbox itself). After a couple minutes, all of these emails from 2014 and 2015 would disappear again and the flagged count would return back to where it was before these emails started loading in. Continued attempts lead to the same result. I then tried searching for emails older than 2016, and it was able to find them and include them in my inbox. After a few minutes, again these messages would disappear from my inbox list. I’m also continuing to find other emails that are more recent that are still missing from my inbox on my iPhone. My iPad Pro is experiencing the exact same behavior.

It’s not the end of the world if I can’t scroll down to emails from 7+ years ago, but I guess the problem is that any time I search for emails from someone that has sent me emails going back 7+ years, the iOS mail app will freeze up as it loads in those new emails and the phone will get hot and battery drain is excessive. If I could just have it so that every single one of my emails are downloaded into the iOS Mail app, it could be assumed that these issues could be avoided. I also don’t like having gaps of missing messages randomly throughout my iOS inbox.

Is there a way to force iOS mail to load and display every single email message that is on the iCloud mail account? Does copying over thousands of emails at once from a different email service (gmail in my case) cause these particular issues? I’d really rather avoid having to get a 3rd party email app, because I don’t want to open myself up to any privacy and security concerns by allowing access to a 3rd party service. Any help would be appreciated. Thank you.