I have also found this very overwhelming. Still do at times. I am always questioning if I am doing too little, doing too much??? I don't think there is a right answer.
I have been trying the cottage cheese/flax oil (Flora organic) mixture every day. I don't know if it is helping but Timber really enjoys it so I figure I would try. Our other dog won't touch it but she has never liked dairy. Timber is 65lbs and I have been mixing 4Tbsp of 1% cottage cheese with 2Tbsp oil with a hand blender until the oil disappears. I think I could up it to 3 Tbsp but like you, I also give some fish oil or krill oil and didn't want to give her too much oil??? I figure she might get something from the fish oil that she may not be getting from the flax.
There is a good book that I found at the library here called Preventing and Treating Cancer in Dogs by Dr. Shawn Messonnier. It describes a lot of different supplements and how they work as well as giving information about dosage so I found it very helpful.
So everyday I am trying the cottage cheese/flax oil mixture
Natural factors IP6 500mg twice a day. (It said to give on an empty stomach but I wrap it in a small piece of processed cheese).
Natural Factors tumeric and bromelain 300mg twice a day
500 mg krill oil or 1000mg wild salmon oil per day
Purica immune 7 (mushroom complex) once or twice a day
AOR brand lactoferrin-250mg once a day (this is a component of colostrum)
coenzyme Q10 50mg per day
Vitamin C (sodium ascorbate) 500mg per day (I could probably give more but am starting low and working up)
Vitamin E 400IU ( I have the succinate (dry) form and the d-alpha tocopherol form but not sure if one is better) I probably give it every other day
The other things I have but do not give every day are
Milk Thistle 250 mg (I give that a few weeks on and then take a break)
Astragalus 500mg (Thinking about giving this at times alternating with echinaceae)
Alpha lipoic acid 200mg (sometimes I give this in place of vitamin C)
Selenium 100mcg (Apparently certain food have selenium like eggs and liver so I will give this if I haven't given food sources)
L-Arginine, L-Glutamine, L-Lysine 500mg each (I haven't quite decided what to do here. I know the lactoferrin has amino acids but I believe the dose for these is quite high in Dr. Messonniers book so I was going to look that up again)
The last thing is Sisu brand MSM with Boswellia 500mg (I am not giving this now for whatever reason but have heard it is helpful for cancer).
I also have brewer's yeast for extra B vitamins which I add to their food and for vitamin A I just be sure and give sweet potato, squash or carrots a few times per week.
If I throw a few canned sardines packed in water, I don't give fish oil or calcium.
For calcium I generally used crushed egg shell which I make and add 1/2 tsp per pound of meat I am giving. Timber also likes goat milk, yogurt and cheese so that also gives her calcium.
Gosh, I hope this doesn't sound too complicated. I think what I try to do is not balance their diet each day but figure it will balance out over an entire month. Just like my own diet, everything in moderation and variety should keep things in check.
If you feed kibble as part of the diet I don't think you have to worry about balancing other nutrients as much. I use to give a kibble breakfast and then make a dinner but timber has decided she just doesn't want to eat dry dog food anymore. I also wonder if it might be harder to swallow or maybe her sense of smell has changed.
Oh, and you mentioned doxycycline. I was given doxycycline when she had the pneumonia like symptoms. My vet mentioned it is good for respiratory problems and MaxaLisa said it has cancer fighting properties. I basically give her that when her nose starts sounding really stuffy. I'm sure when you have a tumor in your nose you are more prone to infection as well.