I've always maintained that sewing is a physical activity, maybe small muscle but still there is hand/eye coordination and other things at play. You can't just get up and play professional sports, and (most of us!) can't do professional sewing results unless we practice practice practice. I just took about 18 months off after daily sewing for years, and it has taken me some time to get back to where I want to be. When I first switched from garment construction to quilting, the 1/4" seam seemed so impossibly small... but after time it starts seeming rather large and accommodating and you can tell if you are right on or big or small.
You said you've tried some of the seam guides, what I had on my vintage machine was an adhesive bandage, several inches from the needle near the edge. I took a piece of graph paper and put the needle through it and used that for placing the bandage. The visible guide of the plastic helped, as did the little hump of fabric. Nowadays I'm a big fan of the 1/4" foot and use it many ways.
It didn't occur to me for years (decades even) to square up my sub-units, I'd just fit them in as they were. When I decided to start checking them anyway, I slowly made the shift away from trying to be very precise with my cutting and my sewing to doing the cut large and trim down. Might as well make it worth my time to check by doing some trims, and it is happier and easier for me not so tense. While trimming can be boring and it feels like it takes a lot of time, I feel it evens out in the next step, no fitting in just nice straight cuts to deal with. If it is just a couple of threads I don't bother but often there is more, especially since I tend to sew scant anyway.
For those that know me, I was not very appreciative of what I originally called "fabric wasteful techniques" but I did a few tests and found that I really don't waste much fabric. Typically whatever I was cutting had a couple inches left from the row that now accommodates the slightly larger pieces. Even on very busy queen sized quilts, I don't think I've ever needed more than one extra row of fabric.
Any time you are doing modern strip piecing techniques or trimming down, you should use a rather small stitch, at least 12 per inch. I think I'm usually in the 16 range -- it makes it really hard with my vision issues to take out stitches so I still try and be precise but know I can be a speed demon when I want to be.
Sadly there is no one "right" way to cut large, we each have our own techniques. Square units are easy for most of us. Pieced triangles are often easiest to have one cut as normal and others cut larger. To help with the trimming I use tape to mark the lines on my rulers. There are many brands of 1/4" tape and I always have a roll in my sewing kit.