Weak and sour definitely sounds like under-extraction. If you want to give lighter roasts another try, aim for a higher extraction. Hotter water, longer contact time, more agitation. These can all help. Don’t be afraid you might be bringing out the sourness more, the sour compounds are always the first to be extracted, just as bitter compounds are always the last to be extracted. Ideally, having a bit of both will balance out the cup and it shouldn’t taste bitter or sour in the end.

The nice thing about light roasts is it doesn’t hide anything in the coffee. Dark roasts have a lot of roast flavour, and that can hide the more delicate, floral, fruity notes that coffee is capable of. Plus if you’re drinking single origin, medium to dark will also mask what makes those beans unique. Not to say dark roast single origin taste no different than a blend dark roast, but it will be much closer to each other than with light roasts. I’m currently bouncing between two beans right now, one from Ethiopia and one from Costa Rica. Now, they also have other things going on, different processes and elevations, but all that uniqueness would get masked if it was roasted dark. So to me, that would kind of be a waste, and there would be no reason to spend a little extra on these beans if they’re not going to taste much different than commodity coffee.

Hopefully that helps a bit.

You can get light roasts to taste really sweet by aiming for a little bit higher extraction, low extraction tends to be acidic. Aim for a longer total brew time. Making the grind setting courser can make it easier to controll the timing.

Trust me, a good quality light roast is an absolute delight.