So, why not build it in Revit from scratch?
I mean importing stuff from Sketchup or AutoCAD is a BIG NO and terrible for your model and performance of everything and causes all sorts of issues as you have noticed.
And that thing you want as a family is not really rocket science either

I understand (and for a newbie like me...LOTS OF ROCKET SCIENCE). However, the original layout was done via Autocad, so I guess that I will need to extrude it on Revit instead. Cue the tutorials!

To be honest I would just use the AutoCAD as a thing to measure from and build it all in Revit, so no importing OR linking anything. This way you are sure it is all accurate and nothing weird like lines that are slightly of axis will occur.

And the process for something simple like this, assuming you don't need all sorts of parameters becasue you wanna change it size (and you don't want to, becasue you were importing from Sketchup) is basically the same as in SketchUp.

Choose the right family template, go with a Generic Model I'd say
Go to the plan view in your family (called Ref. Level)
Start the Extrusion command
Draw the shape you wanna extrude and click the little green tick icon in your toolbar to Finish the editing
In the properties panel tell the resulting 3D shape what height it should be
Save and give it a name
Load into your project

You will find that this process is MUCH easier and faster then doing it in AutoCAD, getting that into SketchUp and then into Revit.

One other thing is that you should really think about if you want it to be this detailed. I mean if you have this in your project 1 time you are OK, if you have this in your project 100 times then all that detail could get you into trouble

Of course I have no idea about dimensions, but this took me about 2 minutes. Added my family as an attachment.

Yup, looks good. I'm actually doing this for a company that will be make it accessible to potential buyers who wish to import it into their project, so I really hope they have good processing power when add multiple versions of the block.

Any good recommendations for those who don't have that capacity? If I "group" the items, would that help?

Yep, don't use the family and don't buy that companies stuff and I'm not even kidding.

But what is it?
My first impression was some sort of prefab concrete hollow brick or something, something like this:

You are correct - the profiles being drawn is for pre-fab blocks. The company's intent is to have the CAD/REVIT drawings downloadable for whoever visits the site, so that visitor can incorporate them into whatever drawings they are working on.

But you say to not create it as a family. What's the alternative?

IF you create this as a family make it simpel, so leave out all the little details, just use the outer dimensions, so a simple rectangle with 3 holes (maybe).
Maybe give people the option to choose which one they like to use, so a simple one and one with all the details.

That being said I really wonder if many people are going to use individual bricks in their model and are you going to provide half bricks of maybe 9/10th bricks (or a way to get those) for when it is not possible to use a full brick?

I have not seen people use individual bricks yet, but maybe that is something that is going to happen, but this means you are going to have 100s of 1000s of these things in your project, so you are really going to have to think about detail.

But whatever you do, do not do the AutoCAD -> Sketchup -> revit route !

Sadly, the company wants ALL THAT DETAIL, so gotta do what you're paid for.