Learn English – Reply to Merry Christmas wishes

This isn't so much a question of English language, but of etiquette. As a native English speaker I find responding to these emails just as awkward, as they sort of put you into a corner, where, as you said, you want to be polite and kind, but don't want to sound unoriginal (which we may fear will be perceived as uncaring).

There are a few options I can think of:

1. Use what you have already written - there's really nothing wrong with the copy and paste vibe; we're all aware that we're basically saying the same things to each other every year.

2. Engage them in conversation: "Thanks, I had a great Christmas and New Year! How was yours?"

3. Go for a slimmed down response: "Thanks, and all the best to you too! :)"

4. Don't respond. This may or may not be appropriate depending on the context. You're not really under any obligation to respond, and it is sort of understood that some people are into sending greetings like this, and others (perhaps those who are more introverted), not so much.

Option 3 is probably a nice middle-ground option, and perhaps the one I'd be most likely to go for.

"Hi John and Mary" is more polite than "John/Mary" but that is probably not the source of the feedback that your emails are bossy. It might have more to do with how you phrase your requests, especially to other managers and peers.

"Hi John/Mary, I haven't received your comments yet on the report. Please get your comments to me by the end of the day. Thank you." = sounds bossy.

"Hi John and Mary, just a reminder that the report is due tomorrow and I need to include everyone's comments - will you be able to get your comments to me by the end of the day? Thanks so much!" = sounds nicer.

Be careful about ending sentences with periods in your emails. For some reason, many people (especially younger people who grew up texting) will think you are angry.