Couple of questions:
- Where are you sending the backups to?
- What are the ISP uplink speeds of your clients?
- Are you using the HyperV version of the software? Or just using the file backup version?

We have several clients with large data-vhdx files. We run weekly synthetic fulls to Backblaze B2 and incrementals the other six nights.
The synthetic fulls take at most 10-12 hrs over the weekend. The incrementals are fast (1-2 hrs.)
But we have two clients with large data files and painfully slow upload speeds (5 mbps). For those it is simply not feasible to complete even a weekly backup over the weekend. We de-select the data vhdx for those clients.
Doing synthetic fulls each week will reduce the amount of time it takes as there are fewer changed blocks to upload compared to once per month.
We have been pushing clients to uplaod to higher speed plans/ fiber as it makes disaster recovery. Lot faster when we can essentially bring back their entire system from the previous day in one set of (large) VHDx restores.
If slow uplinks are the issue and they cannot be increased to say 25 up, then only upload the non- data VHDx’s to the cloud and rely on the file- level ( old format) for data recovery. I trust that you are doing file-level backups to tne cloud anyway so as to have 30/90 days version/ deleted file retention.

Thanks for the feedback Steve. The answers to your questions:
- Sending to Wasabi
- This client has a 25mbps upload which seems to be the standard for Comcast business in our area.
- Hyper-V edition running on a hyper-v host for two virtual machines.

After asking this question, I realized I may not even be able to do anything other than VHDX backups from the hyper-v host. We're doing a monthly synthetic full backup on Friday evenings. It generally takes about 2 days to upload 550 GB. Then nightly incrementals which are generally 5-10GB and take 30 minutes.