If you already have your private BTC, then any trace made cannot be linked to your forum Identity (Dave), You can use a mixer between transactions to make them untraceable.

I do see how making a purchase at this point could affect your privacy as you'll very likely have to reveal some sort of identity to claim your purchase.
You could possibly use a non KYC exchange, convert your BTC to fiat, withdraw cash and spend that as a was to conserve your privacy.

I believe the idea is not to be a total masked man or woman on the internet, but to reduce to a minimum the amount of public exposure you have.