Hello there!
Such fine advice (as always) JMG re Upanishads. May I humbly suggest the Taittiriya Upanishad to start. Any reader/ commentator in this group will find many similarities in this Upanishad with the planes of existence as presented therein. It is also mercifully short and concise;) And ends with the most beautiful hymn of praise and bliss describing (as is humanly possible ) an ecstatic non- dual state.
Best
Jill C