You do have to look at your individual booklet.
A cottons wash is often a really thorough wash. A synthetic or mixed wash is often shorter and can be done on lower temperature.
Eco wash normally relates to cottons. Not all washes you do will need that level of clean.
My machine (Bosch) allows temp adjustment within every cycle, so you can have a shorter cycle and lower temp. For lots of things this would be fine. I’d want to wash underwear on a decent heat (40 degrees) and bedding on that or possibly 60 degrees, but most other stuff would be fine in 30 degrees.
Yes, look at energy consumption. The more a wash consumes the more it will cost.
Do also think about capacity. Obviously some cycles cannot take a full load (that’s usually the case with speed wash cycles) and so the ‘cost per item’ might be higher - that’s relevant rather than outlet the cost per load. If you have to do lots of loads, it might not be cheaper.
But basically, wearing clothes longer, allowing towels to go an extra day or 2 will all reduce the amount you need to do anyway. Kids are horrors for throwing something worn for 30 mins in the washing basket. I’m always returning stuff unwashed to them and we have a rule of 4 wears for jeans and 4 wears for tops which have another top under them and to aim for 2 wears of tops that touch your skin unless is summer or sportswear. Limit the number of towels that get used per week and definitely don’t wash them every day, except tea towels if you use them.