There are good reasons for why both JPEG-XL and WebP exist though.

Oneshot services are for things like scripts that do a thing and exit. Simple is for basic services that intend to run for the lifetime of the system (or for user units, the lifetime of the user’s session).

Create a systemd user unit that waits for the network-online.target.

A script something like:

[Unit]
Description=Startup script
Requires=network-online.target
After=network-online.target

[Service]
Type=oneshot # either simple or oneshot, but sounds like oneshot
ExecStart=/home/<user>/script.sh
RemainAfterExit=yes #if oneshot, otherwise no

[install]
WantedBy=default.target

Edit the template according to your needs and dump it into ~/.local/share/systemd/user/<unit>.service and enable it with systemctl --user enable --now <unit>