Firstly, yes you do need to rest for ammonia. It is the first thing to show up if things start to go awry. Liquid test kits are also more reliable than strips.
To the fish -
Gouramis are territorial and two males should never be kept in a tank this size. Dwarf gouramis are also riddled with disease, either fish TB, or dwarf gourami disease or both. They are both incurable. To be honest, one if not both of your gouramis probably died of one of these diseases.
Otos commonly die shortly after purchase. From the day they are caught to the day they arrive in the shop they are not fed properly. Once otos reach a certain point, no matter how much they eat they will still die. And a 60 litre tank won't be able to grow enough algae to feed a shoal of otos, you need to actively grow algae for them.
Endlers are hard water fish, and two of your sets of readings show soft water though the middle one shows it to be hard. Are you sure the middle set's GH is correct? GH only changes if you are doing something to the tank to alter it such as using calcareous substrate or decor.
Could you look at your water provider's website for hardness to confirm what your GH actually is , please. You need a number and the unit.
Female fighters should be kept either alone or in a group of at least 4. With 2, one bullies the other; with 3 two gang up on one.. With 4, the bullying is more spread out.
Having said all that, the fact that the fish die shortly after you introduce them does suggest something in the water.
Is all the decor aquarium safe?
Do you use any kind of aerosol in the same room as the tank? Use candles or air fresheners?
How often do you do water changes? Do you clean the gravel at the same time?
Do you add any chemicals to the tank besides dechlorinator?
What kind of filter is on the tank?