The unit of measure is just what text you want appended to the value displayed. So it can be anything you want. The value the API will give you is based on your selection of imperial, metric, or hybrid. For imperial pressure is in inches.
There is forecast API but I don’t know how to make it work with the Lovelace weather object.
Thanks for this! I’m just getting started with HA (coming from Hubitat) and this was my first cut & paste customization. Much to learn and looking forward to it.