All is not lost. You can download an installer of an earlier version here:
https://support.microsoft.com/en-us/office/how-to-go-back-to-office-2016-for-mac-15-xx-versions-from-16-xx-e88ed6c8-6fb4-40a1-9540-529000d3d20f
You can cancel your subscription here:
https://support.microsoft.com/en-us/account-billing/cancel-a-microsoft-subscription-c2c6b0e3-cab3-cb98-d83e-c9ad54620530