Explain relic weapons quests to me please

I'm halfway through Stormblood right now and going back to clean up old side quests and activities. I definitely overlooked the relic weapons quests. Poking around, it seems there are quests tied to each expansion? Is any expansion's quest line "the best" for looks or is it all personal preference?

Is there any point in doing the old ones aside from just glamours? I saw each quest had several stages, are they different appearances of the same weapons or something else?

Most importantly, do I have to do the previous expansion's relic quest to access the later ones, or can I skip the ones that don't interest me.