Raising the long-term game – your advice

So, yesterday, it passed exactly 15 years since I started my main portfolio. It has increased by 572%, 13.54% annualized. When compared to SPX and NDQ annualised returns over the same period, it's substantially outperformed the former at 8.68% and slightly the latter at 12.94% (both slightly adjusted for my home country currency). I understand that it's not that easy to generate alpha over the long term, so I am quite pleased with the result. However, I would like to get closer to 20% per annum over the next 15 years. Would you have any advice on how to get to that? Any good learning resources for the more advanced investors?

The portfolio has been long only, single stocks (80% US, mainly tech, 20% Europe), no leverage, 20 – 25 positions. A couple of trades per month, typical holding period around 12 months, unless it's NVDA (2018) or RACE (2016). There were a handful of single digit down years and a few good years, 2020 was +112%.

Should I look to introduce some leverage (levered ETFs?), greater concentration (<10), futures/options, crypto (I hold Strategy and Bitmine), shorts, algo?

Thank you in advance for any (non-personal financial advice) advice.