CRM Revenue Across Channels (Flows, Emails, SMS) Dropped

Hello everyone. 

I am looking for support and clarity as I struggle on one of my Klaviyo clients and I am out of ideas as I tried everything. 

To give you a bit of context, it's a luxury skincare brand that we took over in July, they were with another agency. In September they changed their website, email templates, and then in October the UTM tracking. Plus due to their website migration we have faced a lot of technical issues with the metrics behind their flows dropping and being fixed in October. 

Until August, they were using smaller audiences, 3 or 6 months active, sometimes 9 or 12 plus domain-split segmentation. In August we switched to the 12 months active audience and we saw an increase of the Revenue level. However, slowly starting with September we started to see a decline in their Revenue, even though Open Rate and Click Rate are strong and stable. 

The biggest drop happened after Prime Day in October and after we changed the UTM parameters. The client switched to GA4. 

We investigated everything from Deliverability, Account Set-Up, cookie placement, Full Buyer Audit (logged-in versus Guest), UTM testing (to determine if params stay and they stay).  I see drops at the SMS campaigns and flows level too. 

We are stuck as I cannot rule out what exactly is happening, where should I look and what should I address. I appreciate any help.