What’s your go-to phrase for sounding polite but still being a little sarcastic?

I'll deal with it momentarily. Whether that means soon, or briefly, I leave up to their imagination.

I would suggest… I hear what you say. I‘m keeping track of my appointments in the digital calendar.

With all due respect,

I’m a software developer and something I say at least 3 times a week is “what problem are we/you trying to solve?” It often means “I don’t think you asked the right questions” or “I think you may be doing the wrong thing already”

Or "You haven't really thought this through at all, have you." Honestly "what problem are we trying to solve" is a great re-direct, especially if they didn't start there.

Thank you for your input.

You do you.

As previously mentioned/suggested/explained (to you idiot!)

As you already know… (so don't ask me again!!!)

Thank you in advance for your understanding and cooperation.

“Bless your heart” in the US south can be a back-handed compliment with nuanced meanings.

I understand where you’re coming from, but unfortunately this isn’t up for debate.

I can’t tell you how sorry I am that you feel that way.

“I love that for you”

Quelle surprise 

“There you go”

Manager at a hotel: "How about we try (blank)instead" meaning "what you are doing is absolutely wrong, stop doing it we're doing it this way"

I’ll take that under advisement

As I’ve *already* said…

Bless your heart...

Bless your heart.

I hope you know

Warmest regards

Oh, don’t mind me!

Yes, we’ll have our best people on it as soon as possible.

"Thanks for your feedback."

Actually….

Great. Other ideas?

No, please go right ahead. Don't let common sense or manners stop you.

“If you say so.” 

“Duly noted”. I’ve only used it a handful of times but that’s my professional way of saying “fuck you”, as I’ve only ever used it towards clients being extremely condescending and rude towards me.

'gee, really?'

Thanks for your feedback.

I use the word ‘ambitious’ to mean that the suggestion or idea is completely unrealistic and not reasonably practicable

"You...you go do that."

As per my previous email…

Credit where credit due. Ie not here.

Your question will be answered in the order it was received.