For the short answer, see the comments above.
The long answer is: You can't. There's no perfect way to prevent decompilation of any piece of software.
Of course, there are plenty of good-enough-but-not-perfect ways to prevent decompilation. Any one of them can be broken by someone who is determined enough, but most people don't have three-letter agencies spending large portions of the defense budget on decompiling fangames, so your threat model is rather forgiving.
8.1 is "insecure" because there are easily-available tools anyone can download and click one button to decompile with.
8.1 + anti-decompilation shennanigans is "sort of secure" since a lot of those tools have been around long enough to be broken, but it will still deter a lot of the kids who try the one-button solution and find that it doesn't work without further investigation.
Studio is "secure" because nobody's written a decompiler for studio yet, so your game will only become decompilable if someone else releases a high-profile studio game that increases the demand for a decompiler enough for some hacker to actually make one
Custom engine (no gamemaker, all C++/machine code) is "very secure", because the only people who can hack it are the people who can actually reverse engineer machine code, and those people are too busy with real-life pentesting/reversing careers and/or the cracking/warez scene.
Custom engine + denuvo (strongest commercial DRM available) is "very secure" because the only people who can hack it are the few groups currently cracking denuvo. This cuts both ways, though, since their mission statement is, among other things, to crack every denuvo title. Merely putting that level of (completely hypothetical) security on something paints a big target for hacker groups to flaunt their hacker epeen by cracking it.
Options 4 and 5 are 100% hypothetical, though.
I don't think anyone's made a public fangame yet in something besides gamemaker or MMF. The only exception that comes to mind is co-op, and I'm unsure exactly how reversable construct 2's format is. It's probably somewhere close to studio/custom in security.
Looks like there's plenty of examples of non-GM fangames. Their security probably varies from engine to engine.
TL;DR read the comments above and ignore my wall of text