Here are some tricks I’ve learned while leveling professions:
First, set up a cool toolbar to switch professions. I saw this one in a youtube video. Go to character options / hot bars and choose bar 7. You may like another one, but this is the one I saw used and it’s the most convenient for me.
Once you have the hotbar, then drag the items from your gear list into the hotbar and you can switch instantly between professions / jobs.
So you already have a high level job with gathering gear, right? That’s the best way to gather mats of course. And you’ll see that apparently you have to kill a few of whatever mobs before anything drops, so be patient. Odd. Never seen that described, but I didn’t get Yellow Yarzon Legs for about a dozen kills before they started dropping.
While leveling my Alchemist above level 20, I ran out of things to do. But then I found some “Housing” items.
I haven’t yet worked out how to get Guildleves which apparently are special quests for your profession. These are referenced in some guides I’ve seen, but the guy that was supposed to have them wouldn’t talk to me. More research required there.
But these items require mats that you’ve seen before except for the Dungeon Seedling from The Thousand Maws of Toko-Rak so sometime I’ll have to watch for those. Given the strangled state of housing in FFXIV, I’m not sure how these items will sell on the Market, but you get decent experience from crafting them anyway.
Oh, final tip. Use Stealth when fishing, even if you don’t think you need it.
I was fishing at Fools Falls and after a while the level 25 Fate Monster snuck up on me and killed me. Once something attacks, of course, you can’t change to your level 50 whatever and kill it since you are “In Battle”. Luckily some nice person responded to my shout and came by for a rez.