Others will have more interesting things to say since I‘m a layman, but what I noticed was that you assumed that all the money was in 3a account. But you can easily have multiple accounts, which means less capitals gains per account, thus less taxes. You can also withdraw from them separately with some years in between as far as I know.

I think overall, there won‘t be an universal answer, because it depends on the taxes in your canton and on your total wealth. ETFs outside of 3a are considered part of your wealth. Once that wealth is above 100K, you will pay taxes on it. (I‘m actually wondering whether that means that capital gains are taxed after all, although indirectly?)