You can use protocol data provider for getReserveData and compute the available liquidity (total supplied - total debt). This is the max amount that can be drawn from the Aave pool.

Max amount a user can borrow depends on the collateral supplied. The limit is HF should not be <1 afterwards. You can use getUserAccountData method in pool to get user's total collateral and existing debt balance across all reserves https://docs.aave.com/developers/core-contracts/pool#getuserconfiguration and compute the max amount that can be drawn without dipping HF below 1.

This question might also help.