Advice for 3D modeling peg for sprinkler dripper
I examined and sliced your STL file, and the profile of your threads looks very strange.

It's definitely possible to do very strong, perfectly-fitting threads down to small sizes (at least down to M4 or slightly smaller) using modern inexpensive 3D printers, and contrary to widespread belief (there's a well-known YouTube comparison with a major test fallacy claiming otherwise) they should usually be stronger than threaded inserts against being pulled out. But you need to get the thread profile exactly right.

Most real thread profiles are trapezoidal, but yours peak at points and have round bases. This is unlikely to match the external thread on the part you're trying to fit to it, and it's going to have major dimensional accuracy issues because of the sharp point which can't necessarily be represented in the layer resolution.

I'm not familiar with Fusion 360 so I don't know how to tell you exactly, but most CAD software has libraries for generating threads conforming to standard thread profiles. If you want to do 3D printed threads, you should look at those and figure out which one you're trying to match. Or, if you want to replace the pegs with your own design anyway, just pick a reasonable one for both.

Generally, most modern threads use the basic ISO metric thread profile, even if they're not standard metric diameter or pitch:



Your cross-sections should look roughly like the "internal thread" side of that.