Thanks to both of you!
I have finally managed to make it work. These are the final rules:
Now, on the guest interface, I have Internet connection but cannot access the 192.168.100.* anymore, except the ISP's router at 192.168.100.1
I don't know who's answer to mark as "Solution" since both contributed to the final solution.
Update: my rule "allow_guest_internet" is actually not required. I have just disabled it and it still works as expected. The reason is probably what @psherman suggested in this post. I thought I will need it to allow Internet, but works even without it.
The final touch was to also select the destination zone as @mk24 suggested.
Update 2: as per forum rules: point 6 (multiple posts for solution) I am marking my post as a solution.