It could be a tragic end when there is a bear market and more than half of BTC value drops. If the lender is not reassuring I guess this will work. Isn't something like this already in a Defi platform?

I wouldn't be getting into a loan if I have BTC. What I can suggest is to keep your coins and when ATH, you sell. That's what you should do and then wait for the bear market to come as it always does. You can do whatever you plan with that fiat.