1. Basically I replace heavy/straight/wind/ven/wide volley/rain ect with /ac "action" <t> /ac "Misery" <t> /ac "bloodletter" <t> This is not because omg lazy to click. It's because in later dungeons (coil, chimera, ect) you'll have to silence and if you don't your party might (definately) wipe. So less busy pressing more busy interrupting. It's still good to have BL and ME in your bar sooomewhere because sometimes macros don't like to go off (you have to mash the button a couple many times but that's just how it works with everything) and/or have control of what happens. Reason why you also have them with wide volley is to maximize dps while aoe'ing which could be for turn 4, speed runs, whatever have you.
Using all buff in one macro is not exactly nice because sometimes there's just one buff that doesn't have to come off regardless of you just standing still mashing said button. On top of that, it's nice to know what you're activating (tumults with B4B is asking for eating dirt)

2. Thing is, mages only have to cast sleep once, you have to cancel ballad which includes...two casts (so to speak). There's two ways to bypass this 1. have to buttons for ballad one for on and one for off and both with different messages (ex. Ballad going UP, Ballad DOWN!) which is useful...I guess (I mostly just use TS to tell people) Alternatively buffs can be dispelled by right clicking them on the top center part of your screen where personal buffs are located. This applies to ANY buff.

3. In long encounters which goes first is near irrelevant as long as both have high uptime (very important, high uptime dots=win) Buffs, however only count when the dot is shot/applied not during. When a mob is closing death it's better to use veno bite than wind because veno has higher starting potency and less ticking potency, so if the mob is about to die it's more worthy to use venomous bite for chances of BL and damage. If the mob is literally a shot from death then the answer is obvious, just shoot it till it does (heavy/straight). Thing is, dots have a very high accumulated potency during their WHOLE duration so if a mob dies while having, say 15 seconds left of wind bite, it was most definately a waste.

4. Despite what people have said about dots being applied physically, when playing song it does feel somewhat of a difference (very tiny though). It's supposedly only worth playing when having a caster (which you always do, your healer) and if you can cast it like before battle starts so you don't lose up time (careful though, it pulls aggro.) You shouldn't have much TP issues once you get invigorate. Otherwise, playing TP song to refill yourself is arguably worth it (damage penalty VS waiting for TP)

5. Usually at end game you change gear like a diva. Also, take into account that most end game gear is unmeldable except for special crafted gear which is usually very expensive for fresh 50's. So yes, save your materia unless it's crappy materia, then you can sell it or whatever you wish. If later on, after you get your darklight, you wish to get crafted gear that's the time when it's worth melding but it's a whole different and big topic to be discussed so briefly.

6. Having the combat log on at all times is not really worth it unless you're a healer or something of the sort since most attacks have visual features that allows you to identify rotations quickly.
Tips:
I usually keep all my keys close to each other (buffs: f1-6. heavy shot 1, SS 2, veno 3, wind 4. utility: blunt T, fire arrow F, ect)
Have your limit break like apart from all bars maybe a side vertical bar (you'll use it very rarely but when you do have to, it's like shining in that spot)
Have a macro for focustarget
Always use barrage as the last buff (since it's the shortest and benefits most of having all others up)
Use buffs while GCD is ticking
If needed, help with rain of death (stacking it with virus/eye4eye/bubble really makes nukes feel like tickles)
Communicate with your healers (MP mainly)
Use multidotting for 3 targets or less
Use aoe for 4 targets or more.
Crit chnce is your friend, however DEX is friendlier.
Your relic+1 sucks, so don't rush it.
Be careful when shooting and moving out of aoe, the auto turn can kick you in the nuts.
If you're the silencer of the fight, be aware of what and when you gotta silence (rotations of bosses are very easy to follow) and avoid casting when your turn is coming.
Do keep repel shot and shadowbind, they're handy at times
Use blunt arrow and repel shot for tiny damage increments when BL is down (IF and only IF the fight doesn't need to interrupt, using silence in ifrit will screw people over)