I have not had a go at a 5e Beholder, but I have fought beholders a few times in earlier editions, and that earlier experience is lining up well with what others have said in this thread.

In term of satisfaction as a memorable combat am not a fan. Do not be discouraged by my opinion, but perhaps keeping it in mind will help give you some useful ideas...

You plan out your beholder fight with effective tactics, and it will probably be weird and hard. Weird and hard is not bad (it is more promising that weird and easy), but it is not automatically very good either. The fundamental problem is that "total RNG Rocket Tag" is on the mark. At some level of power and weirdness there are no meaningful clever tactics the players can use beyond employing basic competence.

Sure the PCs will try to spread out so the casters are not easily antimagicked. Sure the PCs will try to not spread out too much. Sure the PCs will try to get close and hit the enemy hard. But how is this so different tactically than how your 6th level party fought hobgoblins? It isn't. The beholder is sufficiently weird and unpredictable that there is no point in attempting cleverness.

If you really want to make a beholder fight memorable, you have to imagine why it is going to be memorable. I would plan on making memorable clusters of minions that the beholder helps for 1 round and then abandons. Make the roleplaying of the beholder and the minions the memorable part. Let the players have their jollies cutting down minions who have their own personalities and are distraught to be abandoned to their fate.

In the final showdown, the beholder is just a glass cannon that will go down in 1-1/2 rounds. Poor thing. Give it some memorable final words about the unfairness of it all.