It looks like normal mode? So Fishron might be not that crazy early hardmode, but if you can't defeat mech bosses either, then why Fishron?
As for mech bosses you have some of the best mage weapons you can get at that time, so, you should be able to defeat them, I think.
I'm not quite sure, but maybe I would try to fight Skeletron Prime there. If you can aim somewhat well enough and focus mostly on the head and generally ignore the arms, I think you should have good enough DPS to defeat him before morning.
Once any mech boss is down, life fruits start to grow, so you can max your HP, with some patience, before facing any other mech bosses. Additionally, you get some hallowed bars, although it won't likely be enough for full hallowed armor with just one mech boss down.
Looks like you're investing into herbs, so, are you brewing potions? You could brew tons of different potions, that could help you a lot in the battles.
Regen, ironskin, endurance are pretty standard ones.
Now that you're in hardmode, you should have access to prismites, that are needed for lifeforce potions - they're awesome!
If you're a mage, you should also make magic power and mana regen potions! Magic power gives +20% magic attack, right? add the crystal ball's clairwoyance buff too, which is +10%, right? that's a flat +30% to your magic attack, isn't it?
And mana regen potion! It's awesome! If you stop casting spells for a second or two, it regens your mana life "whoosh!" it's crazy powerful.
You have that magnet flower, right? I'm actually not super familiar with it, but I personally don't really like the mana flower, even though I like to play mage class (though I like to combine it with melee).
Mana Flower do allows you to keep casting spells, since it consumes mana potions for you, but it gives you the mana sickness, which, I think, caps at -40% magic attack, which is pretty bad. If you sometimes stop casting spells it will go down very quick, but you're using it so you don't have to stop using spells, so, that's kinda contradicting...
I prefer magic cuffs (or celestial cuffs) over them, and on normal, it's totally fine too, if something sometimes hits you - and you often can't dodge everything anyway.
That, or just lean on mana regen potion, though that's only for boss fights.
Also, if you're patient enough, you could try to upgrade your obsidian shield to ankh shield (you can do it pre any hardmode bosses, it just requires some farming, and luck with RNG)
Also, if you fish for potions anyway, you could try to get a frog leg (if you don't have one already) and make amphibian boots, and maybe replace your boots with that, if you want. or replace the magnet flower? I don't really know. but point being frog leg gives you some awesome mobility, and a crazy boost on flying!
(don't throw away the magnet flower though, it's probably powerful, and I can imagine it could be used well when fighting multiple enemies, since there would be tons of mana stars to pick up, right? so you don't usually need to consume mana potions. But I'm not very familiar with magnet flower)