Yep, exactly that!!! Connecting them mandatory would only destroy the pacing of OHC Levels, if you could add them to be optional somehow...
I did some experimenting with Bonus Levels...
I tried to add additional Ending triggers to some Levels so you can decide what Level you are going next but this can break Ending Triggers, no matter what ID or Level you set them for some Reason they always load the wrong Level when you have 2 or more Ending zones so its pointless trying to add them this way.
Have you played the Fan Game "weirder than real Sad Satan"??? It´s here on Itch.io as well. You can´t beat this Game because you are trapped in some sort of Level load Script Error. After Level 8 you are sent back to Level 6 again in a Loop with no way to progressing further.
After checking Level 8 in Terror Engine i noticed this Level had 3 Ending zones, you see now what the Issue is right???
The Dev tried to do the same thing as me and like in my Example broke the Ending zone trigger so you need to cheat to beat the Game.
Btw here is my Playthrough, have added Timestamps so can see what I mean:
Terror Engine can be such a Bitch sometimes.
And with Level Select you don´t even need to connect them in any way, this just means Level Select is mandatory for your patch, no big deal!!!