Only with structured text is there any hope of reuse of the code across brands. Even then it is not 100%. Some checking and manual adaptation will be required.
An exception could be the PLC brands that are based on Codesys.
Maybe you can reuse Ladder and FBD across Codesys based brands.
That should work fine, CoDeSys and most brand specific implementations of it have an option to export parts of your program (e.g. function blocks, functions or entire POU's) in a standard codesys format. Other codesys implementations should be able to import the code.
Porting code to a different brand codesys plc can be more work when your code uses brand specific libraries. E.g. when programming ethernet socket communication, serial ports and the like.