From the sounds of things its time to move on. New alliances, changes of scenery, different titans and different people are ways of stopping the game from getting boring!
As for better loot, it is pretty linked to stronger titans (see this thread: Titan Loot & You - A Data Farming Project! ***PRELIMINARY RESULTS***)
There are heaps of alliances out there, all depending on what you want. From competitive to relaxed. Families to Stand-alones. Religious to Adult only 
All depends on what you want.
As for telling them, well most people just leave a small sign-off note in whatever chat-method the alliance is using. Let them know why you’re leaving & to stay in touch (if thats what you want). You never know, some (or all) of them may follow you to the new home!
Shameless self-plug:
If you do leave & are looking for a new alliance, feel free to message me on Line or @ me here on the forum. My family of alliances is the We Are Groot family.
Alliance Recruitment Thread: