Things I learned on Tactician that I never needed in the OG
Mustadio is OP as hell as a Ninja, because his Seal skills get two chances to hit. He gives you high speed plus almost guaranteed Disable/Immobilize which is crazy clutch. Also makes him a fantastic Poacher.
Speaking of Poach, its incredibly handy in undead fights to make sure downed units stay down when the Tactician effects make the battles draw out.
The basic "broken" setups from OG tactics become midrange setups that do pretty good still, and "ordinary" setups become weaksauce. Brawler Ninjas become a baseline as opposed to being cheese whereas regular Monks and Knights do piddly damage and die too fast.
A high Faith Orator stacking MA with Math secondary is the most versatile unit in the game, and you should build for one as early as you can. Speech takes enemies out of the fight, recruits them for levels or to strip for gear (faster than Stealing), enables Poach farming, allows for early Br/Fa manipulation which is kinda needed on Tac, and high faith Math means you have good hitrates on Induration (petrify), Toad, and Death which are infinitely better than Holy spam. And they can shoot things if they have nothing better to do.
Br/Fa manipulation becomes clutch because Reaction skills are the only way you don't get oneshotted in Chapter 4. Even being a grinder and having a stack of Chantages, I've had fights where I came uncomfortably close to losing because its not hard for the AI to kill your whole team within a turn sometimes. Once I settled on building around Reactions and high Br, this stopped. Shirahadori yes, but also Soulbind, Reflexes, and Vanish.
When your levels start getting into the high 70s in late Chapter 4, Chantages stop being cheese because random battles can be that damn hard. I consider most of my characters to have no accessory slot anymore because putting anything else there makes battles go sideways fast. A few extra damage from Genji Glove just means you take off 30% of the targets HP instead of 25%, while he still kills you in one or two hits half the time. The uptime from Reraise gives you higher damage output than anything else you can slot there - even Haste.
The longer battles mean you'll actually need to refill MP on mages. Chemists are always useful but in OG I phase them out by Chapter 4. Not so on Tac - do not underestimate lategame Chemists and Bards if you're running any mages. I wound up giving Orlandeu Item secondary with Throw Item and he tosses things as least as often as he blasts with Swordskills.
Speaking of Cid, he and Agrias both feel nerfed hard right up until you get Chaos Blades. Swordskills are still great in the vein of "instant ranged damage with no MP cost" but the damage you get is piddly on Tac. A level 70 Agrias with a Rune Blade, kitted for a mix of SP/PA only hits for a little under 200 vs enemies with 500+ HP. Only Holy Geomancer with Ice Sword vs Goblins touches the OG numbers..
Cloud is actually really good now. Limit, Iado, Swiftspell, and a SP focused built and he wrecks house and buffs. He's danged near on par with Ramza for pure utility and power.
Meliadoul is useful! She's either status dancing in the background or walking around busting Armors for big chunks of enemy HP. Cid can do the same and running both is awesome in later story fights. I never thought I'd see the day where, in aggregate, Crush Armor was a better choice than Hallowed Bolt.
Marach is actually useful! Make him an Orator with an MA build, Nether Arts and Reequip. Swap your weapons to use Stoneshooter for range. Use Speech to tank a tough enemy's Faith and then blast them with Nether for ridiculous (if lucky) damage. Far more than Rapha.
As an aside, did you know that if you're careful about timing your Job swaps, you can use Rad, Lavian, and Alicia as your Errand running team for the whole game, and there's enough JP there for them to have every job but Mime unlocked by endgame, while all still being level 9?