I would like a sightseeing mode to look at the graphics.

[Niwashi]

I'd like a feature like this.

I suppose after the raid is over I could go back and take my time at that point (at least until the timer runs out).

I do this sometimes, but besides the timer there's a couple other limitations that may also apply depending on the design of the particular dungeon:

Once a dungeon is completed, the Return skill takes you to the final boss room rather than the beginning of the dungeon, so if there are any one-way portals taking you to the next area, it becomes impossible to access the earlier areas again after that.

Also, if there are any optional mobs, the party is likely to leave them behind. At the end of a synched run, you may have trouble dealing solo with mobs that were intended for a full party.