There are numerous posts about related products in the forums so i just wanted to put my oar in to suggest that maybe this part of VM could be due an improvement upgrade in a later version? It is such an important part of upselling i feel it should be given a priority for improving.
I would love to see any of these as options and I expect others can think of more
1. related product is out of stock - do/do not show it
2. Automate x amount of random related products from category (select category drop down)
3. Select more than one item after typing in the related item box
You can find here a module with many options https://shop.st42.fr/en/products/products-any-ways.htm similar to your request.
It not only work for out of stock but:
-Product headlights only No/Yes
-Set Categories to filter
-Show Products child? No Yes
-prices range with minimum, maximum
-Show Free No, Yes or Only
-Stock : mini and maxi Stock
-or Automatic : Out of stock, low stock, high stock ...
It's not for related product but help to prefilter all the products to render in your shop.
New plugin for related and optional cart products can be founn here now https://shop.st42.fr/en/products/extra-product.htm.
It have no instance limit and can be declared with different settings and styles.