-snip-
Here is where im not sure how do you do it at least in the GUI. Looks like there's some command called
finalizepsbt but im not sure about that. I mean I guess there is some way to finish it in the GUI.
You load the non-finalized but signed PSBT via the same menu: "
File->Load PSBT from..." and there should be a broadcast button there.
BTW, since you're using it on a watch-only wallet, there's no reason to enable the option since "
Send" would be available anyways.
For
finalizepsbt, it should be done in the offline wallet so the PSBT can be fully signed and encoded into serialized raw transaction.
But it can't be broadcasted in the GUI, so use
sendrawtransaction if you do that.