I am also running Linux on Thinkpad (Debian unstable on T440s). As to the battery life, this typically requires some work but I found it is generally possible to get good battery life. I am able to get my T440s to run for total 24 hours with the highest option 9-cell, the built in 3-cell and another 3-cell that came original with the laptop. This I did to be able to work on a long yacht cruise with no extra power and…
I find that I can tune the battery life on a ThinkPad to be pretty amazing, but it is hard to get my change to stick after a reboot or even a suspend/wake cycle. One of the things that seems to make a big impact is PCIe ASPM. But where do you change your config files so it's always on? At least on Ubuntu there are numerous config files that formerly controlled power management settings, are still referenced in Stack…
A Type=oneshot systemd service that is WantedBy=sysinit.target (for early-ish boot) or multi-user.target (for late-ish boot) is the rough equivalent of an entry in /etc/rc.local.
The bootup(7) man page shows how these targets fit into the bootup process.
That said, systemd-rc-local-generator(1) provides backwards-compatibility with /etc/rc.local, so putting stuff in there should still work. Any output or error messages will be logged by the rc-local.service unit.