Where and what stables can I borrow at that rate? Cand deposit BTC into multisig for collateral.
Are you looking for defi only? Then you can check the usual suspects like Aave, which for example offers 5.25% on USDT and 5.2% on USDC, max 75% LTV for collateral.
https://app.aave.com/markets/
But pay attention, the rate is VARIABLE and can (and WILL!!!) change depending on market demands (usually it's going up when FOMO sets in and people borrow stables to invest).
I have done that for a while but it was nerve wrecking, as when the borrow rates rose it needed quick adjustments.
Now I prefer a fixed rate - it's easier to calculate the arbitrage profits and plan longer term.
I have borrowed a huge amount of USDT with BTC as collateral with an FIXED !!! interest rate of 3.4% APY for 90 days, using the USDT to squeeze more profits in many ways.
Unfortunately I can't share the source, otherwise I'd sabotage my cheap borrow rates.
If you need a deep dive into DEFI yields, I recommend this guy's youtube channel. But double and tripple check the platforms yourself, diversify or die (capital wise)!
https://www.youtube.com/c/thecalculatorguy