If you're cloud ele hunting, you might want to bring something that can make it rain. At least feels like it boosts their spawn up a bit. Pretty sure there's something fairly early in hard mode you can craft that lets you toggle rain more or less at will... another one not that much past it that lets you flip the day/night cycle, which is tremendously useful.
Re: Beholder staves and firing rate, they're a bit weird. Sometimes the things will machine gun fire (for shooty minions, anyway), sometimes they pace it out a bit, and a pattern to it I haven't really noticed. Mostly would very much not recommend them for damage, though. They're good-ish for it, but not two minion slots good. You keep one around for the enemy detection until you have other means of that (my current guy mostly just spams clockwork bow around with an accessory that makes projectiles emit light. Reveals pretty much the entire screen I'm on, ehehe), and that's about it.
If you can get to it, I think you could get twisted bark from waking nightmares in the dungeon at this point, though I may be getting confused and that being post mechanical/plantera.* Once you can, definitely do. The critters it summons are pretty mean just by themselves, but they also come with a generic summons buff while they're out. Pretty sexy stuff, heh.
Thaaat said, if you have beholders you can probably survive spelunking in the jungle. Corpse bloom should be loitering around somewhere down there. Kill the thing and a NPC shows up that fixes any concern you'll have for plants for the rest of forever, ehehe.
Also yeah, healing may be more important, but don't forget at the absolute least the unholy glyph's effect is inherited by your summons. It's a pretty nice damage boost if you're looking for one. Can be good to have a mix of enchantments instead of just using one, yeh.
Costs a bit of money, but you're in hard mode on revengence, now. Money isn't going to be an issue for much longer. Current guy's frittering around trying to get up the scratch to stop being one-shot by late game spirit/calamity bosses, and occasionally loses 150-200 plat when something manages to kill 'em, with 300+ in the piggy bank. Cash inflates hard after a certain point, which I guess is good 'cause some of the boss summoning items also sell for upwards 30 plat a pop, and sinking fifty or sixty plat into getting a top grade modifier is pretty normal.
*E: Yeah, double check suggests it's post plantera dungeon. Bit of a ways away, then.