If you have so much time that you could i90 all of your jobs in much less time than the lockouts allow, just make a new character for each job. Just because you CAN have more than 1 job on any character doesn't mean you NEED to. I think its totally silly that anyone would complain out lock outs. I am a pretty casual player (20 hours of playtime on my my intense playing weeks, more often around 10-12) and I have two characters, one has i88mnk, i86pld, i80war, i72sch, and the other has i89 BLM, and a soon-to-be i76 BRD. I'm rarely bored because I cap myth twice, do CT twice, do coil twice, do X modes twice, etc. They stay interesting because I know that I am progressing something different.

Yeah I had to level THM up to level 26 twice (once for my SCH on my main, and obviously had to go to 50 on my BLM), and I will have to level PUG and LNC again for my BRD, but I had different experiences leveling them up with different people, and it adds something for me to work towards. Also, from doing the main scenario quests, you make like 500k gil if you don't spend on anything as you level up. 500k and a new level 50 in about 6 days is pretty achievable with duty roulette decreasing queue times. I'm contemplating starting a 3rd character just to farm the gil!

TL;DR: Make more characters to circumvent lockouts