One thing to consider. When the stocks that are part of a mutual fund drop… then your retirement contributions will be buying them on sale.

Assuming the mutual funds are spread out to minimize risk (1 of the funds companies folds, etc) overall you’ll be better off long term.

As you age you’ll start moving your investments to more stable options (talk to a financial adviser on the specifics for your plans). This way they that won’t benefit from huge gains but also are a lot less likely to be wiped out by massive drops.

In the meantime look at how your funds are doing over time. Not even year to year but maybe every 2 or 3 years.

Sounds like the perfect thing to test and iterate and then use that to have something machined in metal.

Of course with some stronger materials it may be fine. But how resilient will the belts be from the tension/stress and is there any risk of injury when they snap?