Not sure about zrepl, but sanoid/syncoid will keep a separate snapshot for each replication target that has a lifetime separate from your usual expiration policy. So say you setup sanoid to keep 24 "hourly" snapshots, but the initial replication w/ syncoid takes 36 hours. You'd be left with a "@syncoid_HOSTNAME_ISO8601" snapshot, a 12 hour gap, followed by your 24 hourly snapshots. So that snapshot will hold that 12 hours worth of block churn, to allow for incremental sends, until your ISP is able to catch up.

The other option, if you're colocating, is to send a seed drive ahead to the datacenter. (I think there was a startup on here a while back where they'd basically colo your drives in their own JBODs, and then charge you a nominal monthly fee for a VPS w/ those drives passed through as a zpool.) You might pay some nominal fee for remote hands, but it beats waiting for terabytes of data to squeeze through your local cableco's wildly asymmetric pipe.

Back in the day I managed a small office server, backing up via a particularly slow ADSL line. It took about a week to do the initial sync, so I just let it run and in the meantime backed up to an external disk every couple of days.

Once it was up and running most snapshots took a few minutes to sync, always finished before the morning anyway.

Definite +1 to rsync.net, this was >15 years ago but it was always 100% solid, I don't think I ever had any issues. It's nice to see they're still doing the same thing and haven't bloated it with crap!