No, you don’t want to write to anyone. Ignore it and they will never find out and never tax you. Transfers between swiss bank accounts are subject to bank secrecy and the tax man doesn’t have any privileged access.
Even if they do find out, it’s not a crime to have a little amnesia when it comes to tax returns here. For example, pretty much noone (except the completely gullible or if we’re talking serious amounts that can be detected) declares rental deposit accounts, cash in wallet, gold under mattress, bitcoins, etc.
Worst case they bill you for the difference in taxes owed plus a little extra, you pay, end of story.
Best case you get to keep all of it tax free
Bullshi*t. For capital gains OP needs to buy and sell an asset. There’s no asset involved here, or at least someone had the assets and might claim capital gains, but most definitely not OP. He had some kind of profit sharing agreement of sorts. This is 100% taxable income here and they will come for all of it if you tell them.