Is there a way to control the cache mode of Outlook on a machine-by-machine (PC-by-PC) basis in AVD?
Performance wise you're better off leaving cached mode (OST) enabled on the Outlook clients within AVD, especially if using FSLogix. This can be offset by configuring Loopback processing of Group Policy to apply user policies to a machine.
You could reduce the size of the OST by enabling the sync settings to only sync emails in a short period (i.e. 1 day to 12 months or all), as described here: https://learn.microsoft.com/en-us/outlook/troubleshoot/installation/cached-exchange-mode#cached-exchange-mode-settings