Learn English – If any concerns let us know “or” If “there are” any concerns

I think it would be better to mention it as your convenience rather than availability. Here is an example:

It would be convenient for me to have the interview on 5th, 10th or 18th of August. Do please check your calendar and inform me when it could be scheduled.

Here you may mention the dates in any form respective to the proximity.

We remind you... is rather stiff and wooden, almost hostile or at least an unfriendly warning of some kind. We're reminding you... is not as stiff and off-putting but it's a tad unidiomatic. We're writing to remind you... or This email is to remind you that ... would be more idiomatic.