Hello,
Just in case it is needed. I wrote a script that snags all of my purchases very quick (epub). It's just a low-quality quickie, just in case it comes in handy, I've posted it here. No guarantees and such.
https://github.com/redmasq/jnc_swagger/tree/main/examples
I'm probably going to schedule it as a weekly job myself, so as to capture any errata and such. I'll suggest not to use the -forceAll unless necessary since it does cause it to download everything again, which could be hard on J-Novel Club's servers (I'm sure my testing was annoying).
I was having problems with an earlier version of FBReader. I found that repackaging the epub file fixes the issue. The script does this, but it is off by default since the files downloaded works perfectly fine in Calibre and such, and in the latest FBReader (probably was a zip handling issue).
I done it in Powershell since I run Windows for my main desktop, but if bash (Linux), zsh (OSX), or python (whatever) is needed, it's a simple exercise to convert it. For the shell script versions, I need web request and JSON support, so I would probably use something like jq and curl, but they are installed easily through pacman, yum, apt, or brew.
Happy Reading.