CAD / CAM freeware help needed

I have been using sketchup for a while for some pretty basic stuff, but it seems limited or difficult to use for anything outside of architectural designs, of witch I do very little of. 
I went to download Fusion 360 but it appears my version of IOS is no longer supported (10.13.6) even though this mac pro can handle it with no issues. 
Here's where I'm looking for your help. Can anyone point me to a place where I can download whatever version of Fusion 360 that is compatible with High Sierra? 
Alternatively, is there a program you would recommend other than 360 that is free? Specifically, I design things to be 3D printed, if that directs your thoughts. 

Thanks.

You can't upgrade your Mac Pro to a more recent version of OSX?  Fusion stores all your files in their cloud and I doubt they will let you run an older version - fairly certain it has made me update the program before.

Blender: Free and open source.  Great program.  I normally use it for more artsy things where dimensions aren't critical.  There might be a way to use precise dimensions with it, but I haven't figured it out.

OpenSCAD: Free and open source.  Very powerful and has a strong following.  The CAD is script-based, rather than GUI based like Fusion.  If you have done any sort of computer programming, you may like OpenSCAD.  I've only used it for easy modifications of stuff on Thingiverse.  I should take the time to learn it.

Tinkercad: Free, uses a web browser.  Very easy to use.  CAD for dummies.  I've spent hour+ trying, and failing, to do something in Fusion, loaded up Tinkercad and had the project done in minutes.  Probably similar to Sketchup, which I've not used.

Both Fusion and Tinkercad store your designs on their servers, if that matters to you.

Thanks Moltar. No, the Mac can't be upgraded further. In fact, it's been modded to even have that IOS. It's a seriously capable machine, more than capable of handling F360, but that's no longer an option. 
I'll try Tinkercad as I have NO IDEA how to do script-based modeling and Blender is like trying to swim by jumping off a Coast Guard helicopter into the raging Atlantic ocean haha. I actually used it once to successfully model a part for someone, but wow is it hard to use for a novice. I think that's because it's incredibly powerful and capable. 
 

Been planning to design a custom bellcrank for my MTB recently, OpenSCAD looks like it could do the job, similar to a freeware CAD program I'd used to design my 4AGE's CoP deck a few years ago but without so many graphical controls.

Tinkercad can be very capable. It can also be a tedious pain in the ass. It's what I use for most of my projects, but it makes things harder than it needs to be designing out of ready made shapes.

I have open scad and some scripts, but can't figure out how to actually use them.

Blender, like you said, learning to swim in the ocean. It's easier with a digital drawing pad, but still had a fairly steep learning curve, in my opinion.

Those are the big ones. I have fusion 360 and autocad2018, and still find myself fighting in tinker cad because at least I can find all those controls and options, even if it would be easier elsewhere. 

This is relavent to my interests.

I want to slightly modify some already designed things and Fusion looks amazing and very capable but I'm obviously too dumb to learn it on my own. Nothing in that programs seems intuitive at all. I'll definitely be trying some of those others...

This is my beginner course for Fusion (Drafting Level 1, Grade 9), it will lead you from nothing to capable fairly quickly. Help yourself.

https://www.gwellwood.com/subjects/drafting/fusion360-level-1/

Until you said Mac, I was going to say for drafting & model creation, we offer a free version of our software Mastercam (called Home Learning Edition) that has all of the CAD stuff unlocked and the ability to import/export models. 

SkinnyG (Forum Supporter) said:

This is my beginner course for Fusion (Drafting Level 1, Grade 9), it will lead you from nothing to capable fairly quickly. Help yourself.

https://www.gwellwood.com/subjects/drafting/fusion360-level-1/

Whoa. Thanks! This looks much less clinically written than other tutorials I've seen. I think I'll actually go through this.

Can anybody explain what happened to Google Sketchup and evidently it being bought by Trimble? I own and draw in AutoCAD Architectural Desktop 2004 although I am proficient up to version 2018. I want to import drawings from Sketchup into AutoCAD if it is possible and if I can scale it properly. Things like cars, drivers, seats, and engines. Any suggestions?

Yes, for whatever reason Google sold SketchUp to Trimble.  I don't think the free version will let you export to anything AutoCAD will open.  If you buy the Pro version, it should let you export to a DWG or at least a DXF.

In reply to Duke :

Thanks, bummer. My 3D car designs died during the last computer failure that wasn't backed up (dangit). I can do them over except better after the 20 years I've had to think about them. It would have been nice if I didn't have to draw my own 3D driver, seats, or twin turbocharged LS engine. I specifically want to design the boxed flare Prius hyper car for tall people as seen on: Grassroots Motorsports: Photoshop request for the daily . There is a 3D Prius over there in Sketchup that would be nice to start with.

VolvoHeretic said:

In reply to Duke :

Thanks, bummer. My 3D car designs died during the last computer failure that wasn't backed up (dangit). I can do them over except better after the 20 years I've had to think about them. It would have been nice if I didn't have to draw my own 3D driver, seats, or twin turbocharged LS engine. I specifically want to design the boxed flare Prius hyper car for tall people as seen on: Grassroots Motorsports: Photoshop request for the daily . There is a 3D Prius over there in Sketchup that would be nice to start with.

I have access to SketchUp Pro 2022.  If you find SU files that fit your bill, email them to me at the address I use for forum PMs and I will export them to DWGs and send them back.

I've been using onshape. I like it.