This is just not true in most instances (and the 30+ minutes figure is never true, not sure where you're getting that from).
The Brain of Cthulhu, the Eater of Worlds, King Slime, Queen Bee, the Wall of Flesh, and the Golem all have summoning items (which are admittedly consumable but also craftable, except for the WoF's summoner) that can be used at any time.
The Eye of Cthulhu and Skeletron can only be fought at night, but daytime in Terraria only lasts 15 real-time minutes, and nighttime lasts 9 (also, Skeletron's summoning item is not consumable, and the Eye's is craftable as well as littered in chests across the world).
The Mechanical Bosses have craftable, consumable summoning items although they can only be used at night. Plantera's Bulb respawns in the Underground Hardmode Jungle constantly. Duke Fishron is admittedly a massive pain just to summon without creating a specialized Underground Mushroom biome to farm Truffle Worms.
The Lunatic Cultist->Solar Pillar->Moon Lord cycle is definitely a pain since the Pillars take so damn long. The Lunatic Cultist doesn't need to be summoned to start the cycle though, and after a few cycles you can have the materials to create the Moon Lord's summoning item and summon him directly.
I don't agree at all that options are limited, go read some boss fight guides if you think that's the case and you'll see how many strategies are available. Mounts (including a couple with limitless flying like you said), Minecart tracks or Asphalt skybridges for extra speed, teleporters, AoE buffs, tons of weapon options...the list just goes on.
One thing I will agree with you on is that Expert Mode is obscenely difficult (I just recently started an Expert Mode world, even just the Expert Eye of Cthulhu is ridiculous), but that's why it's called "Expert".
The 30 minutes I speak of is actually the fact that your not going to want to summon the mechanical bosses or say skeletron immedietly after losing to them because they are timed boss fights. It's madness to start a boss fight on them without having a fresh night. So if you lose on them, your going to be waiting until the start of the next night, thats about 20 minutes. If your unlucky, you may get a blood moon, so there goes that night, another 20 minutes ect.
Thats what I speak of when I complain about long times to attempt a boss again.
When I talk about options being limited, I speak of the earlier stages and mechanical bosses, likewise. You at best can build a lot of platforms on skeletron and bosses like eye of cthulhu. See the thing is the bosses on NORMAL mode are balanced, but expert they are not due to the waiting time crap combined with there much higher health.
Good luck on skeletron, it took me and a friend of mine 5 times to kill him on expert. Ever since I destroy him on the first try...by sequence breaks, because I outright cannot beat him otherwise. You don't need to play a normal mode world and use post normal mode gear to beat him, but trust me, he's insane. Enough so he just as well be able to one-shot you(and he has killed me 'instantly' a couple times).
Also don't be surprised if you scream in fury at plantera after dying to her 7-8 times like I did and then never want to play expert a second time. There are only a few practical ways to beat her and they are not the usual cheese methods.
The thing is, many bosses can only be fought at night.
They are tough enough(especially on expert) you'll want all of the time you can get.(And outright need it on expert mechanical bosses, except maaybe the destroyer if you get a certain weapon).
So your mentioning them being only summon able at night proves my earlier point about terraria not being able to afford to be very hard due to iteration times being criminally long.
Edit: I will touch on another thing. Yes, you can use a variety of arenas and tactics on the bosses........ on normal mode. That sadly does NOT fly on expert. Tactics that work reliably on normal will get you killed repeatedly.
