If I feel Singapore growth is gonna be stagnant for the next few years, or if SGD is not going to be as strong in the future, is there any ways to store SGD in other currency for a long term?

You can create a multi currency account and change your money through it. You can also used the other currency to buy other equities to improve returns.

U may want to consider multi-currency card. It can help u store but doubt that it will earn you interest as of now. Otherwise, multicurrency account.

Another way is buy foreign stocks

There's this account by standard chartered which I use, USD high account, to store my USD.

There could be some value to put it into safe havens like Swiss Franken CHF

(or Japanese currency?)

However I would discourage currency speculation generally.​​​