DND_Reborn
The High Aldwin
I wouldn't really say it is that complicated, honestly.... But it goes to show that games of this high level are complicated. I mean, look at how many mistakes we've made, and I would think that we count as "very good" at this game!
The first mistake about lair actions is something that is an experience issue. In 4 years of playing 5E, I think I have only used creatures with lair actions twice... maybe... Most such creatures my PCs encounter outside of "lairs", so I don't use them.
IMO, the whole "not using the same lair action twice in a row" is a needless complication to the mechanic.

As for the second mistake, the sunblade, that is something the player should remind the DM of IRL. Besides which, as I noted before, the (fairly) important aspect of the light is sandwiched in between two lines about the range of light emitted:
When I was scanning the item and working on the dynamic lighting for the game, the numbers were what was important do me.
IMO, the highlighted sentence should have been earlier in the text, where it first talks about igniting the blade.
Finally, I would agree we are "very good" at the game, but how much to any of use play at 20th level??? Not many people play at tier 4 IME, so I don't think any of us are experienced at this level of play. It is also a bit slow playing by post, which unfortunately makes us forget details I think we might remember IRL.