Maybe add an option for upgrade recipes for every loader type that takes a loader and some stack inserters (based on the loader throughput) to craft stack version of this loader? Cause existing options don't feel right: stack loader using red loader as an ingredient is strange and makes green loader obsolete; all loader stack is too op; loaders don't stack is just no; green loader+ option makes stack inserters mostly pointless.

Trying to strike a balance on any of these recipes was a nightmare to be honest. And I know it is far from perfect. I don't want to get in the business of saying one choice is op or not, as the way anyone enjoys playing is their choice. I am all for providing some options for different recipe and configuration setups. The real challenge comes with every overhaul option, belt tier mod, etc... Me trying to come up with an optimum recipe set for any given combo just isn't going to be possible. If you (or anyone) is able to provide a full recipe / tech cost / progression and configuration for a given mod set, I'd be happy to try and setup a presets system to select from.

Is it possible to create recipes for every loader automatically based on loader stats? 1 stack inserter can put 14.4 items/sec (without inserter capacity research) so let's round that to 15. So the recipe for any stack loader will be = loader + stack inserter * (loader throughput / 15). As for sprites, is it possible to add quality like marks that identify that it's a stack loader?

That's an interesting concept. It is a significant change to play, and to the code, but very interesting. It will be something I'll need to think on for a bit - after I get a few other objectives complete.