Instead of resizing the existing Widnows EFI System partition of size 100 mb, Can't I just create a new EFI system partition of 500 mb FAT32 for bootloader and linux only related stuff?

Instead of resizing Windows efi partition, I created a new one for pop os as well. So now my system has two different efi partitions. No chance of any messing up.

I tried it but the tool wouldn't let me resize the EFI partition

The part where you want to add 2 EFI partitions is not advisable. It seems that Windows doesn't really like this, of you are dual booting multiple Linux installs it might work. But it is always recommended to use only 1 EFI partition per disk. Hope this helps. :)

You don't necessarily have to resize Windows' EFI partition. You can have multiple EFI partitions.

Sakaki's EFI Install Guide/Configuring Secure Boot