What a problemyour neighbours have, are they paranoid about you in your garden?
If they think you are watching them and I'm sure you are not, just let them get on with it.
Don't even get into a discussion about it.
You have every right to be in your garden doing whatever, and they seem to me to be quite hyper-sensitive about you being there . If they have blinds they can always close them or go and sit inside
Of course you wil be aware of their presence when you are in your garden, you cannot really avoid being, but I am sure that you don't stand and stare in to their place.
So don't spend any money altering what you have and enjoy.Don't let them get to you.
They can always move if it upsets them so much.
Have a similar situation, my neighbour's living room window looks directly into mine, do I care, not at all.I just don't run around in the nuddy and I hope she won't either, but at the end of the day we hardly notice each other, and if we do we wave.
Good luck.