THM lol jk but really if you want to take healing seriously you will need swiftcast for those oops moments where you need to insta raise a downed member. Its not absolutely necessary for pug content (especially if you're so awesome no one dies but sometimes that is out of your control), but you may get some raised eyebrows if you admit to not having swiftcast. I lvled blm to 50 first and thought why not try healing on whm its the logical next step for me! What I learned is I am a gawd awful healer. I was told to find another job when I tried to heal ifirit my first time for the relic quest. Whm can be challenging as you have strong burst healing, very strong aoe dps and when combined can pose some serious mana issues to the inexperienced. If people see whm in party not spamming holy they might complain. If you spam holy and tank dies they will complain. If you spam holy, keep everyone alive and ask for a moment for mana people will complain. Of course through experience, you will learn to fix those things. I couldn't so I hung up my white robe for good and leveled up the tanks. Loved it! But then a rl friend got into the game and wanted to be pally, while my other friend refused to do anything but dps. Against my better judgement, I started leveling arcanist/scholar. Leveling 1-15 was mind numbingly boring. But once I got into my first dungeon everything changed. Although scholar doesnt have the burst and strong aoe healing of whm, I found scholar to be a far more forgiving healing class to learn than whm. Mostly due to your fairy handling like 80% of the healing (in early learning dungeons). It is far easier to switch on cleric stance (ugh so yea you DO need to lvl whm) as a scholar, throw on all your dots, bane em to all the mobs, switch back with plenty of time to heal. I cant tell you how many poor tanks I killed in stone vigil cleric dancing on my whm. As a bonus, once your done leveling scholar all you gotta do is a few class quests and now you have a leveled summoner.
Id like to add that I think youre on the right track by leveling each role. I had no idea how terrible I was as a blm dps until I learned healing and tanking. It has also given me a much needed slice of humble pie. I never rage on my healer or tanks as I know what its like to be learning. Im an "okay" tank and healer. I can reactively get through most content fulfilling those roles. But to be a great tank/healer its all about being proactive. Know when to pop cd/buff before it is needed. Healers and tanks are expected to have the script memorized while dps can quite often improvise with very lil negative side effects. I wish you luck in your endeavors!