Maybe I’m misunderstanding the purpose or goal but wouldn’t this be perfect use case for a virtual machine? I’m surprised no one has suggested that. A one off temporary, easily reverted back to pristine with snapshots sounds like exactly what you would want for testing something like this out.
It’s extremely common in Enterprise where costs for a 100k+ server isn’t the most expensive part of running, maintaining, servicing said server. If your home lab isn’t practicing 3-2-1 backups (at least three copies of your data, two local (on-site) but on different media/devices, and at least one copy off-site) yet, I’d spend money on that before ECC.
DDR5 has built in data checking which is ECC without the automatic correction which might be worthwhile depending on your setup.
Your ECC on the pi i believe isn’t for the memory chip but for the on chip die’s cache for ARM.
For me personally, if my racked server supports it, I get ECC. If it doesn’t, I don’t sweat it. Redundance in drives, power, and networking is much more important to me and are order of magnitudes higher chance of failing from my anecdotal experience. If I can save those dollars for another higher probably failure, I do that.
DNS is a lynchpin of my network (and wife approval factor) which I splurge a bit for with physical redundance of an identical mini computer that runs it and fail over to same ip if the first box fails. Those considerations are way before if the server has ECC. Just my $0.02.
You aren’t giving us enough information to even speculate the answer. Are these Enterprise grade servers in a datacenter? Are these home made servers with consumer or low grade hardware you’re calling servers? Are they in the same datacenter or do they go out to the Internet? What exists between the hops on the network? Is the latency consistent? What is the quality of both sides of the connection? Fiber? Wi-Fi? Mobile? Satellite?
Does it drop too nothing or just settle into a constant slower speed? What have you tried to trouble shoot? Is it only rsync or do other tests between the hosts show the same behavior?
Give us more and you might get some help. If these hosts are Linux I would start with iperf to do a more scientific test. And report to us some more info.