oh shoot how do I get the true true ending then? I went to surface and killed both guardian and king (both seemed like happy ends compared to what happens when you go deeper into the dungeon) defeating the throne guardian actually resulted in a longer credits scene as opposed to others that just had a single name pop up (i.e no special thanks). And by doing this I unlocked the ability to save anywhere at any time. In addition there's a new loot area right at the start point that lets you fight a slime so you can finally get the adult slime loss scene.
His website has hints;
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This is the parts that are relevant.
メイン攻略ルート
右手側の道を進み地下から脱出END → 地下魔王ルート → 砦ルート → 真魔王ルート → 王城ルート
→ ちっちゃくなっちゃったルート → 街から世界脱出ルート
・世界から脱出すると、回想ルームが開放される。
これが今回の目標だ。
・果ての怪物を倒すと周回時にアイテムを引き継げるようになる。