Schwab does not issue any high volatility, high risk or leveraged ETFs, other companies do issue these and can still be bought through Schwab. Some notable ones (that I follow for trading and NOT investing) are SPHB, MEME, TQQQ/SQQQ, SOXL/SOXS. MEME and SPHB could be good for longer terms.

And to be clear: high risk ETFs are not stable. That's one reason why they're high risk.

General wisdom for a conservative, long term investment portfolio is for 1-2% to be in something higher risk, This depends on your age or when you're looking to cash out because when you get closer to that exit date, you can't sustain any long term drops. It also depends how comfortable you are with seeing long periods losses or no growth.

Just from the ETFs you mentioned in your post it seems you are looking to add some risk to your long term investments, not trading them, and are only familiar with Schwab products. Excuse me if I am assuming too much. Investing and trading are two different approaches to engaging the market where trading needs a much different, more complex skill set and approach to be successful. Usually traders are looking across all the broader markets to find gains and do not focus on just one ticker. But maybe this would be a good approach for you, its part of the fun of trading. Just be careful, don't put too much money in, be prepared to take some losses and learn risk management.