If you use the environment variables I mentioned, you can get any officially released version you want with 9999. In a sense, I would say that it would count more as a test result, than one with an ebuild from an unofficial source does. :]

Which version of nvidia-drivers are you using (415.25 here at this time)?

Also, could you grab the terminal output from a session where you experience a crash, and throw it into a 'pastebin' of some kind (and link to it here)?

I haven't actually tried Wine-Staging for FFXI in a while. It causes my whole desktop go wonky (windows turn black and stuff like that). It may be caused by some setting in the prefix that I have set in the past, however, though I think this one should be relatively clean. I'll need to look into that more with time at some point.

Here are my usual flags:

app-emulation/wine-vanilla-9999::wine was built with the following:
USE="X alsa fontconfig gecko gstreamer jpeg mono mp3 ncurses nls openal opengl png ssl threads truetype vkd3d vulkan xinerama xml -capi -cups -custom-cflags -dos -gphoto2 -gsm -gssapi -kerberos -lcms -ldap -netapi -odbc -opencl -osmesa -oss -pcap -perl -prelink -pulseaudio -realtime -run-exes -samba -scanner -sdl (-selinux) -test -udev -udisks -v4l -xcomposite" ABI_X86="32 64 (-x32)"
CFLAGS="-march=znver1 -O2 -pipe"
CXXFLAGS="-march=znver1 -O2 -pipe"

app-emulation/wine-staging-9999::wine was built with the following:
USE="X alsa fontconfig gecko gstreamer jpeg mono mp3 ncurses nls openal opengl png ssl (staging) threads truetype vkd3d vulkan xinerama xml -capi -cups -custom-cflags -dos (-ffmpeg) -gphoto2 -gsm -gssapi -kerberos -lcms -ldap -netapi -odbc -opencl -osmesa -oss -pcap -perl -pipelight -prelink -pulseaudio -realtime -run-exes -samba -scanner -sdl (-selinux) -test -themes -udev -udisks -v4l -vaapi -xcomposite" ABI_X86="32 64 (-x32)"
CFLAGS="-march=znver1 -O2 -pipe"
CXXFLAGS="-march=znver1 -O2 -pipe"

Just remembered that it's not too long from now, when I did experience some rather random-like crashes myself... but they were very rare, so I'm not too certain it could have been what you are seeing. It very well might have, however.

I haven't seen them in a while now though, so I'd definitely encourage you to at least try with rc5 via the 9999, or wait for the 4.0 release for a re-reassessment. :]

Also, if possible, try Wine-Vanilla as well (which I mainly use for FFXI, so there might be things happening with Staging that I know not about).