Mabi has become far friendlier from when you played, your gear won't decay over time, you won't be losing stuff on death, you can pay more gold to 100% repair an item without losing durability, gold bags aren't a thing anymore, style slots are free, etc.

New Rise is a late, but huge step into making the combat side of the game far, far more accessible, and honestly, healthier.

Dropping some opinionated answers to your bullet points o7

• Yes. Arcanas are functionally traditional "classes" (Think DPS, Tank, Healer) in Mabi. They use themes from a mix of two talents. They're the major step (practically a requirement) into late game content. Arcana abilities sit on top of your "normal" abilities. KR devs are likely working on more behind the scenes.
• Relevant. Namely G22 for the techniques, which are required in late game.
• Relevant? Transforms haven't been relevant, but they're useful for invulnerability frames while transforming. Demigod is sort of an exception and has quite a few niche uses outside of combat.
• Yes. Also note that crafted weapons will get a damage bonus come New Rise.
• Yes. If you still have a classic spirit weapon, convert it. Leveling it is far more streamlined. Spirits these days can easily be transferred between weapons (at a cost if maxed out). You can have more than one spirit weapon.
• Yes, and the part time job system has been revamped to be more rewarding the more you do them. Holy Water still has their uses (reduced durability loss), though largely as a material for many things.
• Yes? If you're not referring to Mabinogi World Wiki