Applying rubber to the boot won't help unless the added rubber is stronger than the boot itself. Otherwise it will just let the boot crack. You won't get stronger rubber without heat. Forget about it.

Split boots are junk. Even if you do manage to get it glued in the presence of the grease (not an easy task), they won't work nearly as well as the OEM device. Forget about these too.

Silicone spray may indeed help, if the cracks are not very deep. But if there are any cracks at all, you're in for a new boot sometime. Best do it before it gives out, or at best you'll have to dismantle the joint to clean out the dirt.