Hello,
According to https://www.epson.eu/viewcon/corporatesite/kb/index/1018:
"Compatibility details for Epson DX8400
Your product is macOS Big Sur compatible. The product driver is available via the Epson support website.
Printer Driver: Please use v10.85 or later.
Apple Image Capture Application (ICA): Yes. Click here for support on using Image Capture with your scanner."
The printer driver v10.85 is the key. Since the support page for the Epson Stylus DX8400 does not appear to include this driver, you may want to try what others have done: Check whether you can locate a Printer Driver v10.85 elsewhere (for another model, with Big Sur cosen as the operating system on that printer's support page). You could perhaps begin with the link provided by luigi7119 on page 1.
EDIT: Just noticed that you had already done exactly that, and that the printer is working. Very good!