I’m still trying to learn about this too, so I may not be explaining this properly.
The domains using Exchange servers are receiving the most TLSRPT reports.
One domain is using the hybrid on-premise/online Exchange server. The other is just online/365. Those domains immediately started receiving TLS reports on DMARCian as soon as I setup my MTA-STS policy and TLS reporting.
My understanding was that Google products supported MTS-STS from April 2019. So I was surprised that I was not receiving any reports from those domains using Google Suite. However, after 20 days or so, I am starting to see a few reports from those domains now as well.
However, as you say, the TLS reports are from senders attempting to connect to your SMTP, so the mail server of the destination domain may not be the factor I should be looking at.