DEs dont use mount and fstab, they use udisks2 which works with polkit, GUI prompts or rootless.

Using udisksctl prevents a ton of breakages.

I dont know about how autostart files work anymore, I always thought just place stuff in ~/.config/autostart but now those dont work anymore on KDE, sometimes.

I think you use your init system for that. If you go fully rootless, you can create a user systemd service that mounts the drive.

mkdir -p ~/flashdrive

cat <<EOF > ~/.config/systemd/user/flashdrive-mount.service
[Unit]
Description=Mount flash drive on /dev/sda
#After=multi-user.target

[Service]
Type=oneshot
ExecStart=/usr/bin/udisksctl mount --block-device /dev/sda --mount-point /home/$USER/flashdrive
RemainAfterExit=true

[Install]
WantedBy=multi-user.target
EOF

systemctl --user enable --now flashdrive-mount.service

Not sure if After=multi-user.target and WantedBy=multi-user.target twists the space time continuum or something.

I am always kinda confused by those targets, as you must state one.