Invest what you can afford to lose OP, because many investors have taken such risk in the past but they lose everything while those investors that invested what they can afford to lose succeed at the end, because they understand how the decentralized investment work before investing. Since you are making income from the altcoins why can't you continue investing on the altcoins, because you are making income from it, show that the altcoin is among the potential altcoins that is bringing income to their investors and traders. For you to invest the funds on USDT, I will advice you to invest them on Bitcoin and hold for long term, because that is the best strategy to make huge amount of income from Bitcoin investment which I know you will definitely succeed from Bitcoin in the future.
I will still insist on saying that while it is advised by several bitcoiners that it is good to always invest all you can afford to lose as a principle towards mitigating risk management in Cryptocurrencies investment, investing in Bitcoin should not be solely based on such principles. It should just be a little bit of a friendly advise for anyone who's new into the crypto space and is obsessed with the ideology of making huge profits from investing in Bitcoin.
However, investing in Bitcoin is not just all about putting in a sacrificial amount of money, but putting in a lot of efforts towards understanding the market volatility, strategic investment and portfolio diversification. Instead of focusing on a single portfolio, building a diversified portfolio during investment is key to managing risks that could emanate from a failed investment.
The volatility of the crypto market is also one aspect that many people fail to manage properly, due to their fear of being lost out of a particular trade or investment, they end up investing a huge capital on a needless investment with unrealistic expectations, resulting to disappointments in the end.
Ultimately, the amount of profit you should invest in Bitcoin is totally up to you as an individual to decide. Like you spare out some percentage of your profits into your savings account, so you can spare some percentage of your profit to your investment account in a bid to secure a financial independent lifestyle as an individual.