From my understanding there 2 types of Dao. Heavenly or natural Dao (which is objectively exist and natural to world) and Personal Dao (which created by cultivator when they achieve Immortal realm and independent from Heaven). Former is Dao person born with, later you form from your life experience, goals, personality etc. And personal Dao can include pretty much any other Dao, but they still will be only part of it.Why do people that have reached a very high rank not study other dao? Like why does the sovereign not study the dao of creation when it could reduce the weaknesses of his destruction dao? LB said that if we evolved our dao while not having Tri-Dao we will be barred from ever attaining it, but aren't gods supposed to be anomalies that did the impossible? Can a god only be the god of one dao? Is that why people with more than one dao need to fuse them into another dao? I know that there is no actual answer for why Gods cannot practice multiple dao but what your theories?
For example, TW had obsession as personal Dao, but it includes body, space, elements and others. So I think Immortals and Gods can freely study other Dao (both heavenly and personal), but they will be just a part of their Dao (though they will work in some unique way, like only Sovereign destruction Dao can kill even Gods, but not TW who also apparently can use destruction)