It might be reasonable to reward tomestones if you do it enough, but 5 tomestones is way too high.

That would mean that 20 fates gives 100 tomestones, which is about what you get for doing a combined leveling roulette, expert roulette, and main scenario roulette. You can burn through 20 fates extremely fast, even in max-level zones like The Tempest.

Making it more like 1 tomestone for a fate is much more reasonable.