There are two other good ways (that I can think of) for getting seals:

1. Adventurer In Need bonus for leveling roulette. Keep a watch for when the roulette is offering the bonus to healers.

2. Supply and Provisioning hand-ins. One item is requested for each crafting or gathering class you've unlocked - although you don't need to personally craft/gather the items. It also gives you EXP for the relevant class, so it's possible to level them without actually using them! As the classes level up, their requested items (and rewards) get higher as well. (Plus 1000 bonus seals per week from the challenge log for handing in ten requested items, and again for 10 HQ items.)


As a new player on the free trial, I leveled everything up to the Lv35 cap, and I had so many seals from crafting hand-ins that I didn't know what to do with them. Bought every piece of gear and then started stocking up on ventures.