GA sessions splitting at giving (secure trading)

Question

I have an issue that's happened since we replaced the UA code with GTM and now run everything through tag manager.

Everything was working fine with analytics using plain UA. We could see ecommerce, user journeys etc all in one sessions however last October we changed to GTM. Initially we left the UA ecommerce code in place as our Devs did not have the capacity at the time to replace that.

That cause an issue where everything after a certain page become a new session. I tried setting the tracker name in GTM to blank, which I've been told fixes the solution, but still 90% of those who gave (It's a charity) would start a new session one a process giving page. Note: There are about 10% who appear not to be affected.

We then thought it might be because the GTM code and the UA code wouldn't accept each other as the same session, so we recently removed the UA ecommerce code and replaced it with GTM.

This broke the ecommerce altogether and I only managed to fix that yesterday. We also had some virtual page views for checkout journey which were also only fixed yesterday but were working fine on UA until we implemented GTM.

The issue seems to be that at the last virtual page on the form people are having their session ended. Then there's this processing page and then the thankyou page with the ecommerce code on.

I have looked through the user journeys prior to us having GTM and I cannot see any pages in that journey which I can't see currently (by this I mean I don't think there are any other pages that I should be making sure have GTM code on and thus breaking the journey in two) but even if there were, sure since GA sessions last 30 mins, it shouldn't necessarily matter if there were a phantom page.

Should I be setting the Tracker ID to blank again? I'm at a loss as I've been fixing all these problems since October and my brain has melted.

Edit: After a bit more research it looks like secure trading might be the issue. Does anyone know how to overide secure trading making a new referrer and keeping the sessions the same?