Hey Alex!
I saw some Syn Studio templates on your recent artwork and I was wondering if you're taking classes with them? They're located literally minutes away from where I live but I don't have any plans to try em out yet - just curious as to your thoughts. The alumni work from Syn is absolutely breathtaking!
So I always preface my thoughts on sci-fi and mecha by stating I am a mechanical engineer by trade. My day job is working on rocket engines. I like mechanical things, I understand them, and good design makes me happy inside.
Now, most of the designs you'll see on Pinterest are futuristic in the measure that they don't have a direct real-life equivalent. What makes the GOOD ones stand out is how grounded, realistic, and appealing they are when compared to the next best thing we have in real life - modern mechanical engineering. Beyond the basic design theory that also applies to fantasy or any imaginary designs (big medium small, shape language, etc.) there are rules in the design language that make up mecha.
To create good mecha, you need the right knowledge and vocabulary - you can't make shit up and expect it to pass, it's like trying to fool people with incorrect anatomy! You can look up basic mechanical features that go into product design. Chamfers, bevels, counterbores, ribs, fillets, trusses, etc. All those things are patterns that fit together to make a mechanical shape look believeable.
Knowing about how mechanical parts are fabricated helps in designing an imaginary machine that looks believable. Is this part folded sheet metal? is this one CNC machined? is this one molded or cast? What material is it? What is its purpose? and so on.
You might find that if your designs look "gimmicky" it's because you have a narrow or limited visual library, and you don't really know why you're placing features. I'm not asking you to become a rocket scientist to make better mecha, but I think studying products that are in a certain aesthetic to your mecha will help. If you study and copy a crawler excavator from reference, you'll learn design elements relevant to industrial mecha. If you study a motorcycle/superbike, you'll learn design elements relevant to sleeker, faster looking mecha.
I get specific in my mech design because I have an idea of how hydraulics, machined frames, and protective plates actually look like, because I've studied them a lot. To skip engineering school, you can also expand your visual library by making Gundam kits, or miniature plastic robot kits of any franchise. I consider those part of my training - for a few years I was REALLY into them.
Here's an example of my work - I spent 13 months assembling, detailing, and painting this robot from a simple kit into a detailed diorama with innards and robo-guts (gallery link here).
Even if you don't paint these kits, just putting them together counts as a study session - you'll get familiar with the repeating patterns, rounded features, angles, edges, and general aesthetic that goes with that specific sci-fi universe.
Bottom line, study mecha to get better at mecha. There's no other way! Hang in there 
