Is there a way to respond to all emails in a specific folder at once
rather than opening and responding to each one at a time?
Sure, create a rule that automatically replies to any message (ie, don't
give it any conditions) and have it reply using a specific template (that
you've already created which has the text you want to reply with) and do
not enable the rule. You definitely don't want it running automatically.
Then, in that folder, do a run rules now and run that rule.
You will likely want to test with a smaller folder that contains maybe a
single message you've sent using an external account to verify it works first.
