First off, it looks good as it is. Well done! Also, it works and that's the important bit. Some things which help me:

• use equality constraints instead of repeating measurements for radii and lengths
• fillets can be added after creating the main shape, this simplifies the sketch
• if you know this will be mirrored, draw a quarter and mirror the extruded body in part design (you could use multi-transform here)
• use formulas, spreadsheets or reuse named distances if you want to change things later
• make extensive use of the Part Design workbench but sometimes dabble in another workbench because when they click you'll unlock a new set of combinations and turn FreeCAD into a sort of swiss army knife

Congrats on your first model 👏

In cases where you can know ahead of time your part is going to have a lot of repeated dimensions, i.e. it is symmetrical in one or more axes, or you know all the screw holes are going to be X diameter, or whatever, abuse the hell out of the equality constraint. That's what I do.

Select a bunch of lines and hit E. Now all those lines will always be the same length. Constrain just one of them to whatever length you need; all of them automatically follow suit.

Select a bunch of circles and hit E. Now all those holes will always be the same diameter. Constrain one, you get the idea.

This makes the task of, "Oh crap, I changed one tiny parameter and now I need to find it and change it again 3/7/15/etc. more times" evaporate instantly.

The sketch is a sketch and not a blueprint or engineering diagram so don't worry about how a screenshot looks. (It looks fine and I can almost read it, FWIW.)

My advice is just general advice for parametric CAD. It's hard to learn, but so awesome to use.

Get a good set of calipers and reverse engineer everything you see. If something seems super awkward to make and you feel like you are building a domino tower, stop and attack the problems from a different angle.

Fully constrained sketches are super important at first. Constrain everything until you learn what bits need to be adjusted later. The goal is to build solid components that can be adjusted later while they are part of a much larger assembly. I have a subset of "stock" components that I share across my models. If I change one measurement in a core component, all the other models that use it are adjusted almost automatically. That might just be for Fusion 360 though, but it shows the power of understanding constraints and how they trickle up through your projects. If my sketches weren't constrained correctly, the results may be wildly unpredictable further down the workflow.

I have been working seriously in CAD for about 5 years after I decided it was going to be my pandemic project. TBH, I still don't know everything and probably never will.

I think the sketch could be salvaged quite a bit by moving your measurements around. A lot of them are stacked on each other.

Symmetry is a lot easier than it is made out to be. On square shapes like you have you would essentially select opposite corners then your origin (0,0) it would automatically make your square centered.

Equality constraints would also clean this sketch up a lot since you have a lot of areas that are identical.

I'd be happy to look it over if you could send the file somehow

Edit: Spelling

I'm currently following this guide on YouTube. He's really great at teaching FreeCAD.

https://www.youtube.com/playlist?list=PLWuyJLVUNtc3UYXXfSglVpfWdX31F-e5S