So...

It's basically just Peaca Abyss with more absurdly powerful mobs, a lame boss, and the option to buff mobs and nerf players? Are the rewards in this instance even that good?

If you're only doing basic-level, not really. You need to get A-rank reward tier or better for a *chance* at the good stuff. The main 'good stuff' are:

• AP (it's bad at Basic, though)
• Alban Knights Emblem. Judging from the patch notes, they work like upgradeable totems, but you need a lot of runes just to make a complete one to synth into another. Do keep in mind they break if you fail trying to synth Level 4+ Emblems, though. The cap is level 5 for any one emblem type, far as I heard. I don't have enough runes to upgrade mine, so I can't really tell you if the stat benefits are worth it.
• Alternate source of Crusader EXP.
• The (very rare) coin drops in supply chests. I was lucky to bump into a chest that has a Gold Alban Knight Coin. It's worth 100k when NPC selling it (and it even allows you to deposit said gold directly to bank, due to absurd value, though you obviously get less due to bank deposit fee)

tl;dr version: generally nope, though opinions may vary if you can handle S-tier or Hard-rank stuff.

Due to the whole "3 times a day" restriction (and I can't run it more than once on my comp due to memory issues), this is going to be a huge pain to do research on.

In all honesty I don't think those gold Coins are rare, every single time I've ran into the Supply Room I've gotten one :/

I was only quoting directly from the main page's article on it. Running into the Supply Chest in and of itself is rare enough. Ignoring the lucky streaks, there isn't enough raw data to disprove the actual rarity of the gold/silver coins yet.

So DevCAT wasted all their time doing this, rather than doing a proper Archery Renovation... I have a feeling that veterans will just get bored with this content eventually, just like with 99% of everything else in Mabinogi.

I want to know if there's some sort of a prerequisite for entering the instance. I heard completing G19 isn't required, and someone else also said the accompanying quest requires at least cumulative level 500.

No pre-request, the quest that comes along with it I'd say is minimum 500 levels, and the reward adapts to up to 10000. Intermediate from 500~1000, 1000~???? advanced, Hard from ????~10000, and 10000+ gets a heroic stone.

From something I had at least some hope on, its extremely evident that they really didn't put THAT much effort into this as it would seem, and it further confirms that they pretty much really didn't want to do anything for Archery renovation that was worthwhile, and this is filler content.

I'll still run it because I'm a sucker for pointlessly 'challenging' garbage.

I don't even know if: I am doing something wrong, I am weak, or the content is dumb. I've always thought mabi was a "fight smarter, not harder" game. This rolls along and, even in basic, I am being plowed by multiaggro nonsense that one-or-two-shots me.

So... is this just bad content or am I being unfair?

"fight smarter, not harder"

Maybe before a lot of updates that came after Fighter.

There was a time where you actually had to think before attacking, now you can pretty much facedesk in front of the monsters and they'll drop dead. :/

Instant Aggro + Advanced Heavy Stander + absurdly high stats was pretty much DevCAT's solution to producing "challenge". Pretty lazy and bland method to save them from making a competent AI.

Especially during around chapter 1 stage of the game was mostly where "fight smarter, not harder" was, but post-Genesis swapped it around. You can't exactly "fight smart" when the above takes a huge control of Mabinogi's combat.

I remember when Ghasts first came out, people ran away like headless chickens after being torn apart by them xD; But yea, the Instant Aggro + Auto Defenses + High Prot/HP stats are so common now, but they can all be easily taken care of. Somewhere I think this is why there's an absurd amount of Grandmaster Bards nowadays, that +50 Music Buff Duration is basically equal to +1minute when talking about Lullaby... in one minute if you can't clear out most a room these days you're either a new player, or have been slacking/standing around dunbarton far too long for your own good.