I'm trying to figure out if I should just put all focus into Scrap Mechanic or if there's something even better

It looks like Trailmaker has the most workshop support but can't afford it right now

Here's my summary of what I tried so far:

Planet Explorer - boring, convoluted - modeler takes too much time to master
and there's a steep learning curve

Terra Tech - Very limited (ie; mechs can't walk..have to be on roller skates)

Brick Rigs - Ok for car combat lego bashups on a small scale but doesn't have anywhere near the scope of the other games yet.

Emperion - Takes too long to harvest up the parts you need (unless they've added an unlimtied sandbox mode)

Stormworks - Limited instructions and maps are mostly comprised of oceans - Easy modeler to use

Scrap Mechanic - Easy modeler very similar to Stormworks but very buggy and easy to have to ditch out of a game due to a glitch losing a model you forgot to save.

From the Depths - kudos on all the help the game offers but they tried to overcomplicate things, the modeler is a mess and most gameplay takes place in the ocean.

KSP? You can build 'road' vehicles is you want, though I suppose technically manned rockets are vehicles as well.

BESIEGE!

I have yet to meet anyone who regrets purchasing this.

And yes I know, I don't like In Dev either, but this one is worth it.

I guess Jalopy is too simplistic.

SimplePlanes: https://www.gog.com/game/simpleplanes

No idea if it's "best", but you do build vehicles.

Folks frequently mention Cataclysm DDA as a great vehicle building game.

Planet Nomads has a vehicle-building mechanic. Not sure, if this is what you´re looking for.

For £7 I could forgive it being in dev. Wishlisted for now, looks interesting.

Banjo-Kazooie: Nuts & Bolts?

for now, Trailmakers is my favorite - https://www.playtrailmakers.com/

.
.
I"ve been wondering about that...it seemed like it was so early in development that I've been stepping around it

.
. .
.
Got it =) .
.
That's an awesome game...not a builder but I like the sense that you're going on a road trip; the developer needs to patch it though as I haven't had a successful game yet where the game hasn't glitched and I had to start from scratch again (usually at the first motel). .
.
Have it but haven't tried it yet...just planes isn't it?

.
. .
.
I can whip stuff up in Silo, Hexagon, Blender than these bulder games but I don't know how to program my own
game environment to play with my models in .
.
Have Planet Explorer and Emperion...is there much difference?

.
. .
.
I'll wait for the developer to have an installer...I'm not good with building source files
.
.
Can only afford PC
.
.
From my research this looks to be the star of the show but aside from the massive content in Steam workshop you're pretty much on your own; community and devs operate through a Discord channel and there Discord channel is pretty useless
.
.
One other that may be on par with TM is Astroneers which has taken the No Man's Land Idea and mixed it with a builder (Astroneers appears to be a much more user friendly Emperion)

Was "My Summer Car" already mentioned?

https://www.youtube.com/watch?v=LNkLhED5Elw

Beam.ng Drive seems good. Not sure if Spintires lets you build your own vehicles but I wanted to mention it.

Astroneer is not a builder game as such. it is an explorer, base builder and survival game.

edit - yes, you do "build" vehicles, but they are pre-fab thingies you print out whole in a 3D printer, apart from adding seats and energy sources, that's it. You unlock different vehicles by doing research and strange thingies, but the focus is on base building and resource gathering, not on vehicles. (though you do need to print out a rocket of some sort at some point, as different planets have different resources).

It is also far removed from No Man's Sky, as there is only one single solar system, no "mystery" or quest nor are there any wild life. (by the way, it is Empyrion. Empirion is something completely different....)