I think this is a very fair critique and as a fan of martials I get going to bat for them. The antimagic cone, despite the name, does shut down the melee PCs the most.
That being said, I have run a Beholder fight as strictly a boss battle and for that I absolutely leaned into terrain and the Beholder being super intelligent/knowing its own strengths. There was a lot of narrow tunnels and verticality. I also stole some material ideas from a 2E beholder supplement on DMs Guild, mirrors that reflected eye beams and stone that disappeared and reappeared when exposed to an antimagic cone or not respectively. The first half of the fight the party was running down a zig zagging hall smashing mirrors to stop eye rays from coming down at them from around corners. When they got to the end of the mirror zig zag hallway of death they realized it opened out at the top of a huge shaft with disappearing-rock platforms and an acid bath at the bottom. Some clutch teleportation from a bard and toughing it out athletics checks from the barbarian and the party won but I beat them down pretty good from full and they had a paladin (albeit one of mercy w/ low Str who ended up being the primary target of Telekinesis) to buff their saves so it could have probably gone worse.
I also rolled the Beholder's eye rays out a day ahead of time and then came up with a flow chart and targeting priorities with the PC's sheets in hand. Beholders are super intelligent super paranoid ultra xenophobes, once they see a foe for a second or two they can assess the best wat to defeat it imo. I keep the randomness to represent the ultra paranoia but I think planning/being tricky and brutal is a very important aspect to the Beholder encounter mythos.