reviewing models

10 times lower without losing any detail sounds good. :)

You can use grid fill if you want. I prefer doing it myself because I have better control over the mesh. The point of the triangles is that even if you use grid fill, after applying the subdiv modifier you will end with more faces then you need. The top cap of the cylinder is flat, but we have the vertical loops that make up the sides of the cylinder running across it both on x and y. We want to keep these but not all the extra faces so the arrows "funnel" large to small density faces allowing us to get rid of some geometry.

https://photos.app.goo.gl/zgbieQuytTuiDvFz5
What the models will be used for determines poly count.
real time apps like games and vr usually low poly where as high quality renders or animation require higher poly count for level of detail and realism. I model mostly high poly but just try to get rid of any geo that isn't contributing to the models LOD. A single high poly model isn't an issue, but if your scene has a hundred props in it and they are all high poly it will strain your system.

Good!

K,
Delete default cube and add a 12 sided cylinder.
As it sits we have 14 faces so far, the twelve that make the tube plus the two end caps.
Add sub div modifier at 2 levels, 1 loop cut at top and bottom to get our shape back and a small inset the top and bottom faces brings us up to 60 faces. The top and bottom faces are still n gons, turned into quads we now have 76 faces. We have Two loop cuts top and bottom. Shade smooth. Now apply the subdiv modifier. In edit mode select all horizontal loop cuts except the bottom and top two. You should have at least seven selected. Dissolve these suckers. Your poly count just went way down.
Now there is still a lot of faces on the top and bottom cap.
Top view, vertex mode, about five loops in from top join center vert to upper diagonal verts on either side creating an upside down triangle. mirror that in exact same spot at the bottom. Now if you alt select and shift alt select the two edge loops that join the bases of the triangles and dissolve edges you got rid of a bunch of faces while maintaining the integrity of your cylinder. You can take it across x axis as well. Experiment and you can take it further.
Anyway, if the surface isn't curved it doesn't need a bunch of edge loops, just a few at the top and bottom to maintain the bevel, hope this helps.

I know how you feel. :)
I had a look at a few of your models. Nice work.
My only suggestion is you can improve your poly count.
I use blender as well. After applying the subserf modifier there will be lots of un neccessary geometry you can and should get rid of. Don't have time now but later tonight if you want I can explain in detail with examples, pics.