When I used to work, I'd wear a shell jacket for rain only when it was cold.

If it was warm, I wouldn't really care how wet I'd get because you'd get inside your jacket anyways. So I'd have a garbage bag over my short sleeve jersey and shorts. Most cycling clothes are quick drying polyester anyways so when you arrive at work, just dry off with your towel to have there and hang your cycling clothes to dry. If you get soaked going home, no big deal. You're going to shower anyways.

If it was cold, I'd wear a shell jacket with lots of ventilation and long sleeve cycling jersey for insulation. I'd unzip the armpits and zip up only the top of the jacket but leave the bottom open. The front zipper is one of those two way zippers.

All my cycling clothes are cheap and not necessarily name-brand or cycling specific. Same with the shell jacket.