For posterity, a more public update:
With some help from
@Byte Knight via CQ II running on ZTerm, I finally figured out the download issue somewhat. Zmodem worked perfectly fine once I slowed the rate to 9600. I was told I can go faster with hardware handshake enabled, but I completely forgot to test downloading a file running at a faster rate with hardware handshake on. (Trying to download using Zmodem at 57.6K without hardware handshake resulted in "Data CRC error" and "Hdr CRC error" issues aborting the download.)
Downloading files are kinda moot when I can't un-stuff the SIT archives on the SE. It's a shame this machine is so hobbled because it's the most interesting one in my collection, and the only functional vintage Mac at the moment. (Hoping to get the Classic up and running again here in a few weeks after getting its logic board reworked and cleaned... some oxidation causing weird problems, despite having it completely recapped and ultrasonic cleaned back in 2020.) The SuperSE can't load high-density (1.44MB) floppies (pre-FDHD/SuperDrive), can't run some software due to the accelerator, can't work with the HD-20 emulation mode I typically relied on for larger file transfers. And, most frustratingly, it can't run any version of System 7 with the accelerator installed... it just completely ignores the fact its installed and runs on the stock hardware (68000, 4 MB RAM instead of '030 and 16 MB) despite having the drivers.
A lot of the uses I initially wanted to use this machine for require System 7.
System 6 isn't bad for writing and other basic uses, but browsing the web (rendering
any HTML documents and/or simple web graphics), using more advanced software or working with higher-quality graphics (e.g. TrueType fonts) is nearly impossible.
Anyways, enough complaining for tonight.
The previous owner got a lot of use out of it and its younger sibling, my 1994 PowerBook 165 awaiting restoration. I just wonder how he managed to open StuffIt archives.