Handbell skill is only counted when you have a handbell equipped, whereas geomancy skill is always applied. That's the only difference. In practice they're equivalent.
Thank you very much.
Can I get away with not using the Renaye ring? I've noticed that most sets of gear in these guides use quest and mission rewards without at least providing an alternative. In fact, what would I do for my black mage for a Fast Cast ring without the Weatherspoon ring, for example? I feel like every guide for every job should at least consider that...
Absolutely you can get away with not using it! Once you get enough JPs(Gifts), Dunna, Merits, some of the AF/Relic/Emp pieces, Solstice, augged JSE cape(WKR) or Incanter's Torque(this gives 20 skill combined 10 geo/10 handbell) you'll be able to easily hit the cap of 900 and blow it away anyway; nothing after 900 skill will matter though. You don't need all these to hit the magic 900 combined skill I was just listing all(iirc offhand) the pieces that help with it.
As for BLM fastcast ring and for GEO too you options are prolix ring, Veneficium Ring if using GS & I believe theres a newer fastcast or OCC Spellcasting ring from Zitah or Ru'ann but might be wrong.
Edit: BTW if you don't have a Dunna yet
DO NOT use Filiae Bell(15 skill) over Nepote Bell(+5 Geomancy (not skill))!!! The Nepote Bell will buff you buffs way more!!!