I do not assume Outlook 2021 will sync with a Yahoo hosted calendar in order that’s positively not the reply (transferring your calendar occasions to Yahoo).
Your finest wager can be to do like I do and use a separate account someplace to host your calendar (and contacts), sync to each the brand new account and your e mail account in Outlook, and transfer all of your calendar occasions to the brand new calendar within the new account. Then simply sync to the brand new account in your iPhone.
I’ve a Google account for contacts and calendars however my important e mail is hosted Trade by Workplace 365.
In your case, the most effective place to get that new account might be Outlook.com since that may work nicely with Outlook on the Mac…and it is free.