Looking for some advice on how to shave off ~5mm from the vertical part of the arm of the chair (it’s stopping the chair from sliding under a table).
Welcome to the Bunnings Workshop community @hg1423. It's a pleasure to have you join us, and many thanks for your question about adjusting chair arms.
Do you have an electric sander? If you were accurate, you could cut a few millimetres off with a handsaw and then sand to finish. Since there is only 5mm to remove, I'd be inclined to take the slower route and sand them down the 5mm. There is less chance of making mistakes that way, as sanding is more forgiving. With 60-grit sandpaper and an orbital sander, you should have these adjusted in no time.
Just so you are aware, you'll be removing the finish on the chair and will need to replicate it again to match. Another option instead of adjusting the top of the armrest would be to trim down all four legs the 5mm. At least then, you won't be adjusting something in full view, and any mistake you make will be somewhat hidden down low.
Please let me know if you have any questions.
"Another option instead of adjusting the top of the armrest would be to trim down all four legs the 5mm."
That's what I'd be looking to do.
Here is another option, just add a 5mm packers to the table legs, this way you won't need to modify the chairs.
Love your thinking, @JoeAzza! Sometimes the simplest option is the best one.