Galactus can just take away his powers...
Galactus getting high 1-B upgrades soon for lifebringer
Also, thanatos is missing his intelligence for some reason, and galactus is nigh-omnicient. Galactus has shown multiple times to be immune to death and even as far to come back from it through sheer willpower. I do not see any of Thanatos' hax working on Galactus before he is dealt with. Galactus has more senses than Thanatos can comprehend via the power cosmic, atomic creation, manipulation, destruction, and restoration won't work on galactus via the power cosmic, Galactus has domain over all energy via the power cosmic, they both have the same immortality types, matter and mind manip do not work on galactus as he is made up of neither, galactus exists in all dimensions, even dimensions that thanatos would not know or comprehend ( can provide scans), galactus can also create portals, good luck possessing galactus with only 3-b hax, galactus has no soul to possess, galactus isnt alive and can resist 2-B death, Power Cosmic>Psychokinesis, Galactus energy manip>thanatos manip>, the rest are self explanatory.
Galactus is non-corporeal, is an abstract, reality warping, time manipulation, complete control over energy, his matter manipulation is sub atomic>atmoic thanatos, physics manip, perception manip, empathic manip, memory manip, gravity manip, power manip...
The things in the first bit paragraph are going through all Thanatos abilities and if galactus can get around them. Galactus has infinite stamina vs nearly limitless stamina for thanatos. Did I miss anything?