Again, I think you are not understanding that when you install a 100 amp panel with a 100 amp service behind it, you have TWO 100 amp legs, which each return down the neutral, unless they are part of a 240 circuit, in which case they return down each other.
Two 12.5 amp heaters? One on one leg, one on the other.
The welder of course is 240v. That is a powerful welder you have there. It is drawing current from both legs simultaneously.
All the 120v stuff is drawing from one leg or the other, and returning down the neutral.
So if you have a 50 amp panel you have a total potential of 100 amps. If you have a 100 amp panel you have a potential to deliver 200 amps, assuming you can perfectly balance the load.
In my youth I was a film electrician. We often had to struggle to get the load balanced within a few percent. One day we were on a huge project and were using all of the generator's capacity. Just plugging in a 5k lamp (4.8amps) was enough to shut down the whole system.
Do what you think best. Copper is expensive.