President Reagan met with his Cabinet to discuss a package to strengthen relief for debt-burdened farmers, but a group of rural banks said the response still is insufficient. The farm debt crisis has been at the top of the agenda of several recent Cabinet-level meetings. Agriculture Department officials said the Administration will announce today details of new federal guarantees of existing bank and cooperative loans to financially troubled farmers if banks agree to lower the interest rates on them. But Thomas Olson of the Independent Bankers Assn. of America said banks should have to write down neither principal nor interest to qualify for federal guarantees, which he said should total $3 billion.