it's hard to describe, but basically in lancer it's very limited, very strict and so you can't just imagine something, you have to pick from a list of existing stuff and so Idk if there's any system that is made to allow for custom made stuff, that work around vibes and that could maybe be all theatre of the mind? but still with rules and overall a system
Mutants and Masterminds is a superhero system but it's so freeform that you can basically do anything with it
Salvage Union, pilots are not tied to a specific mech. You can even pick up abandoned mechs and pilot them with no training. There are a variety of mech, each of which can be customizesld with different configurations.
You can try Aether Nexus, it's lighter on rules, although I don't know if that's what you're looking for.
My personal favorite to recommend is always Fabula Ultima. You can flavor the classes that you mix and match as mechs or mech parts/frames/weapons. It also has Techno Fantasy Atlas with a dedicated Pilot Class, but you can skip it if you're going to play as mechs anyway (Tho it might give you more cool ideas). It's also all theatre of the mind, as it is based on JRPGs, so the combat is quick and easy to follow.
I haven't played it, but heard good things about Maharlika RPG, and supposedly has good customization. But if you want something more rules loose and potentially theater of the mind, Amour Astir: Advent is a PbtA system. Less mechanics, more flavor, etc.
But tbh I've never had an issue with Lancer not being "free" enough. The license system means you can build a mech from parts from a half dozen others, and anything can be reflavored or tweaked if it doesn't quite meet what you're feeling.
You may like Mechahack it's got classes but is much more free form. I also know of Beam Saber but I have not been able to play that yet.
Celestial Bodies! It is this!
I disagree with your interpretation of Lancer, with every RPG flavour is free and your mech can look however you want and the key art is just to give an idea of how the mech may look. The starting mech, the Everest, lacks key art entirely for this exact reason.
That said maybe something like Beam Saber is more your jam? Its a Forged in the Dark game so it'll probably depend on whether you like that or not.
it's so good and so underrated
Oh hey Eric! Any luck on those 3d6 beer glasses?
Celestial Bodies
Amor Astir
Aether Nexus
Beam Sabre
Remnants RPG
I've been kinda curious about trying Celestial Bodies for a bit. Nobody I know likes GMless games but this looks like it'd be trivial to run as a GM, and the mech system feels so silly but interesting.
Metallurgent
https://www.metallurgent.net/book-and-resources
Apocalypse Frame
https://www.drivethrurpg.com/en/product/485431/apocalypse-frame
It is fine as a gm'd system, I did a few sessions and it worked well.
Battletech destiny, A time of war, or Mechwarrior RPG.