I wouldn't worry about it then. Do what you need to do with the account this year, odds are the US government will never notice. There have been literally no reports of the IRS reaching out to any fully non-compliant non-residents on the basis of FATCA information. I suspect that likely won't change for sub-seven-figure amounts.
If indeed you "intended to give up" US citizenship when you became Swiss in 1999, and never "acted" as a US citizen after doing so - no tax filing, voting or passport renewal, basically - then for all intents and purposes you haven't been US citizen for the past 30 years. You simply chose not to spend money documenting the fact, and you supplied your bank with an SSN because you were required to. If you ever do pop up on the IRS radar, document relinquishment and you'll be retroactively cleared of any obligations because it happened prior to 2004.
If you could only renounce (i.e. you were not able to prove that you relinquished in 1999) then it would be hypothetically advantageous to do so immediately, before you dissolve the account.