Planets with "abundant" chtholon are more common than those with "abundant" aetherium even though chtholon is described as a "rare" resource in the Beta 1 announcement. They are definitely much more common that planets with "abundant" chronimium. This may have to do with the ratio of dark to light nebula in the current beta.
Reduce the chances of core.chtholonAbundant within planet classes in CoreEnvironment.ars, or reduce the frequency with which these classes occur. Alternately, change the ratio of bright to dark nebulas in FallenWorlds.ars
Trillum and chronimium are needed for starships but also to build advanced infantry types, defenses, etc. Ae & Cht have no use besides ramjets. Empires that are mostly non-nebular have little incentive to enter nebulas.
It might make sense for certain prestige defenses or structures to require them in order to give more empires an incentive to try to get toeholds in dangerous nebulas. Maybe SRMs and/or other constructions, once implemented, could use of nebular resources.
Ramjet autofacs and yards that are not heavily trade-reliant are intrinsically less productive than starship equivalents. This is because they have to operate an additional structure. Standalone ramjet planets need structures for CG, trillum (to make CG), hex and aetherium (and maybe chtholon). Compare to starship yards, which only need CG, trillum and hex (and maybe chronimium). I believe that this decreases the total labor pool by splitting labor among more low % structures. Moreover, there is no biosphere, high-population-cap nebular resources planet class that is equivalent to an underground, ocean or earthlike world (just empyreal, which is equivalent to a desert world; these worlds are not very good at high TLs). This means a standalone ramjet planet will almost always be running a habitat structure too.