Depending on the bacteria species you can storage at glicerol 80% for at least four months. For example S. Aureus. You could make some tests with your isolates to test the time they can survive in the glicerol 80% at -20°C.
Indeed, 50% glycerol slants serve as effective storage solutions for bacterial cultures at -20°C, leveraging glycerol's cryoprotectant properties to safeguard cells from ice crystal formation during freezing and thawing. To prepare them, mix glycerol and sterile water equally to create a homogeneous 50% glycerol solution. Inoculate the slants with the desired bacterial culture and let them grow before adding the glycerol solution to cover the growth. Seal the slants and store them at -20°C to maintain viability over months to years. To revive cultures, streak cells from the slants onto agar plates and incubate under suitable conditions. Although glycerol enhances viability during storage, it's vital to handle cultures carefully and adhere to proper storage and revival protocols to ensure their viability.
I think using glycerol 15 to 30% is much enough to preserve the bacteria at -20°C. in fact, high concentrations of glycerol may affect negatively the bacteria storage.