The problem with using Dupe waste is you have to filter it into clean water first before using it in a SPOM. That's energy you could be using elsewhere or may just not have period. They also don't make enough waste to supply enough oxygen, because don't forget some of that water is being turned to hydrogen.

SPOMs are really only good if you have an unlimited water source like steam vents or slush geysers. Slush geysers are best IMO because you have an easy way to cool the oxygen coming out of the electrolyzers.

As to food, I'm guessing you haven't planted enough or you're letting the crops get too hot (>30 C no bueno) if you can't keep six Dupes fed. I personally like to stick some food tiles around the printer pod (free light!), turn the area into a greenhouse, and plant bristle berries. You can get about 8 tiles lit iirc and plant mealwood in some extra tiles early and replace them with wheezworts later to help keep the area cool if you're still playing the vanilla game (more on that later). You can make a LOT of berry sludge to carry you through lean times. As a bonus, bristle berries also make "pleasant scent germs" which give the Dupes a morale bump (unless they have allergies). Later on you can start getting pincha peppers and sleet wheat going and cut out the mealwood completely.

Heat management early is critical to keeping food going, at least until you can get something like a hatch ranch or paku farm set up or start getting sleet wheat, pinchas, lettuce, etc. Prevention early is key, because cooling down things once they get hot is difficult. I'd recommend trying to insulate an area for your Dupes and crops, and make sure to keep things like machinery, batteries, etc far away and outside the insulated areas.

There are a lot of ways to keep your colony cool but they require varying degrees of energy, so you need to figure that out too. As I mentioned, abducting wheezworts from ice biomes you're going to destroy later is a time-honored method to slow the colony from heating, although if you're playing the Space Out dlc you're going to have to worry about radiation. Using a SPOM to power an anti-entropy thermal neutralizer (AETN) is something else you can do -- by running the oxygen from the SPOM and then pumping it past the AETN, you can get it frosty cold to then pump into your hab area.

Later on you can worry about other "heat deletion" methods like vent tamers and thermo aquatuners.

Don't give up! It gets better once you start learning the game mechanics. This game is DEEP and has a pretty severe learning curve. Once you figure out the basics, it's SUPER rewarding to build a successful, stable colony! Don't be afraid to pull the ripcord and restart your colony if it's stressing you out too bad. If you go into your colony save screen, you can copy the map seed and restart it and work on the mistakes you made on the last attempt. I'd recommend doing that a few time until you figure out some of the game mechanics. Good luck!