Why it works
People who'd never accept a recurring calendar invite happily answer a friendly "when are you free?" — because it hands them the control. You get the regular cadence you want; they get the flexibility they need. The ask is identical every quarter, so it's a perfect static email.
Make it yours
Use it for partners, mentors, key clients, or anyone you want to see regularly but not rigidly. Adjust the cadence to monthly or twice-a-year depending on the relationship.