In my experience, going from 1 dog to 2 dogs is like 1.5x the work. Its more work but if they get along, they play and socialize together and that makes it not really double the work. and of course some stuff like house cleaning the fur etc has a baseline minimum anyway so extra fur is only a bit more work.
but somehow, going from 2 dogs to 3 dogs is like 3x the work, unless maybe you have extra help like older teenagers who are resonsible and engaged with the dogs or you have more of a working setup like a farm, sports dogs etc. With bigger dogs, its hard to transport 3 dogs safely in most passenger vehicles like cars and light SUVs. It becomes hard to take care of annual vet visits in one single trip. Boarding costs go up if you need to travel and its harder to find vacation rentals that allow more than 2 dogs, esp giant breeds.
Something to consider separate from day to day lifestyle.... with your proposed timeline, you would be ending up with 3 senior dogs one day. 2 of whom are breeds known for shorter lifespans and sometimes costly health issues. There is the emotional aspect to consider of the high probability of losing the dogs within a short timeframe, maybe within a year of each other. Plus the financial aspect of vet bills for 3 large dogs, everything costs more for giant breeds and seniors can have those bills add up.
Waiting a bit longer, maybe till your younger dog is 3-3.5 years old would reduce those issues.
In general, if the breeds are not known for same sex aggression AND the individual dogs are fine with other dogs, neutered males are probably going to work out OK. Intact males and neutered males can have issues with each other.