If you double a memory block with realloc, the memory may be relocated to a different address (at least on MacOS). Then all pointers pointing to the old block will be invalid. This can be addressed by adding new blocks to a list. It will be more complex than a minimal arena allocator.

Furthermore, a typical arena allocator only grows but doesn't shrink. A deleted item from the memory block will still hold the memory and will not be released. You will need a more sophisticated allocator for deletion. For fixed sized items, memory pool may be an option.