In addition to all you said, you mentioned your car...do they also pay you for your commute time? Of course not.

Say you have a 1hr each way...2 hours/day, 5 days/week, 48 weeks/year, $100/hr: $100 * 2 * 5 * 48 = $48,000. WFH literally just saved you $48k of billable time.

Buying a 4k monitor (that you can also use for non-work activities) on your own dime is probably reasonable.