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Make an invisible/blank spr, actually its a little bit time consuming because you need to replace them all

Of course you can just replace the sprite in the .grf you are using. This will make the client show the new sprite (which can be anything) for the selected job.

Might look weird in some scenarios since player sprites are of course the most detailed in regards to movement and animation indexes.

But if its as simple as that, for example using a poring.spr/.act, just add it to your server grf and name it the same as for example the swordsman spr/act and follow the folder structure. Your client (if data.ini is setup properly) will read your server grf first, notice that Swordman has a poring sprite and load that, instead of the normal sprite.

1. Copy the monster.spr and monster.act you want.
2. Rename the monster.spr and monster.act that same with the job that you want to replace
3. replace the original spr and act file of the job you want to replace

Did you try to update your grf files?
https://github.com/zackdreaver/ROenglishRE

I used this one, i didnt encounter any problem using 2019 client

These are BMP images, you can find it in your GRF  and translate by editing the picture or you can find a data folder with these files already translated...