I’ve heard the rule of thumb is that mass transit will basically always take about as long as driving there

From personal experience this is true with some caveats - it doesn't take into account waiting times, or that mass transport will never stop exactly where you want it to, or that sometimes you need take multiple ones to reach your destination. All those things add time that potential passengers take into account before deciding "I'll take the bus" vs. "I'll drive" or "I'll take a uber".

So if you want your mass transit to improve it’s always worth it to do it at the expense of drivers: they’ll become your riders.

Yup - and that's what a mayor here did in the 90s, to encourage the usage of the bus system. For example certain central avenues got bus-exclusive lanes, and car transit in the leftover lines actually decreased because of that.