how to generate hard surface kitbash

In my opinion, I think you are spending your time in the wrong place.

Your first image is great, 12 products. Most 3D artists can adjust these to the other 429 pieces you made by moving the vertex points as they need for whatever width, so personally I think you have spent a lot of time just adjusting the sizes when the artist will do that anyway for their own purpose.

If I was in your position I would make more kitbash kits with 12 products instead of 35 variations of each product, this way you can output more content, piece together more kitbash kits and sell more content with less small tweaks and producing more items. For example instead of making 1 apple with 100 variations of that apple, I would make a fruit stand with apples, pears, bananas, etc. Within each fruit type I would maybe do 4-5 variations to break up the repetition.

Resizing doesn't make a model unique and in general this item is not a kitbash, it's just a few simple similar details.

Here is an example of real kitbashes that can be used in the works: https://vitalybulgarov.com/3d-kitbash

a very good teacher as well

Thank you all
So far I have understood my biggest problem.
But please answer two other questions:
1- What would you recommend for the parts to work well with each other?
2-If you were a buyer, what factors would you consider for buying such products?

1. Load a base mesh (it can be an arms/legs/head/skull or whole body/car body/plane/ship, etc.) and create 5-10 parts for each section (as a section there can be muscle/bone/body parts/wing). The main thing is that everything should look solid and smooth. In addition It would not bad to increase the level of knowledge in anatomy/mechanics and hydraulics.

2. The main factor when choosing a kitbash is how much it can speed up the work. In a good kitbash it should be intuitively clear how it parts can be used.

keep working