You don't have to keep your coins in a centralized exchange to be able to access a stablecoin like USDT you can do so in some none custodial wallets. Some none custodial wallets that have USDT are Unstoppable and Cake wallet, you can do you own research for other none custodial wallets. The strategy is quite tricky because you can be tempted swap your Bitcoin back and forth when it rises to a certain level or dip to a certain level. Swapping your Bitcoin during dip is an option that is open to us, we can either take it or leave it. Do you think that it is a good strategy or a waste of time?
Let me emphasize that this strategy is not important for investors that plans to DCA for like 10 years and more because Bitcoin will reach many ATH before then. It is good for investors that plans to sell within a short time or a few years.
Instead of doing this trading, cause what you're describing is basically trading, so instead of using DExs to do this, it's better to use CEx, I mean, the fee is small and you can both buy and sell in this for easily and more comfortable in spot trading, no leverage, small fees, make profit both in a down trend and up trend.
In my honest opinion, if you want to preserve your Bitcoin, hodl or convert to stable as you've suggested, and wait until it is time for bull run in the market, no need to be swapping back and forth, or swapping and each check point, it's unnecessary.