Another possible connection point is the "power distribution block" that's tucked in beside the battery itself.
View attachment 125787
In the 2019 fuse allocation book, it's listed as F150/1:
View attachment 125788
....
As for the negative ... well, it gets complicated.
If you connect directly to the starter battery's negative post, you're bypassing the "master disconnect" that's located by the accelerator pedal.
If you connect to the frame, then the disconnect will still remove the starter battery from the system (helpful when servicing airbags, for instance), but would leave the "house" systems still alive.
If none of your house loads are using the frame as their negative connection, then it bubbles down to *only* the charging (alternator) system is using the frame.
If it were me, i'd lean a bit towards using the frame, not the starter negative post, as my connection ... since it would leave the Master Disconnect still properly operating.
(you may tell the service writer about it, but will that info filter down to the poor mechanic)
As a help, you can add a label (such as wrapped on the cable at the disconnect) warning about the 2nd "ground" connection.
--dick