I am an 11 year tau vet and I have been using the new 'dex since its release.
Poster spect_spidey is right on his #3. Dual guns are better than a
TL gun, in the 7e meta, especially with
ML support. 7e is more about volume fire (as
GW puts more models on the table in Formation detachments) than 5e.
On his #4 though, I disagree. Marker Drones are far better (though BS2) than Pathfinders, with higher T and
JSJ for safety. To make them better, you could go with a
Baby-Sitter HQ -
Dual Missile Pods, Drone Controller (they now use the Cmdr's BS5!

) And always, Irid Armor to make it far harder for an opponent to score Slay the Warlord. The Baby-Sitter should always purchase two MarkerDs of his own, making a 6 pack when he joins a
FA Marker D unit.
With you only owning 2 other Crisis Suits ... I'd go dual PRs or dual
FBs ... depending on the amount of armor you face. At 800 points ... yeah you could face
AV 13/14, but I'd doubt it. Volume fire to kill the mostly infantry lists you face under 1k points.
At this point, you mostly need advice on purchases:
1. ebay - 3 more Crisis Suits to complete a
MSS/C&CN supported unit:
a. Vre -
MSS/C&CN, maybe flamer for Wall of Death when charged. Do not put the
MSS/C&CN on the Cmdr. as you're sacrificing the Cmdr's BS5 for the 'Vre's BS3.
Cmdr - dual guns (I recommend dual PRs) Irid Armor and VRT. That PEN chip is good, too.
3+ other suits, dual guns. Skip Target Locking them as they need to erase whole units and the splitting fire of 4e and 5e doesn't hold quite as well in 7e.
Maybe 2 Gun Ds for ablative wounds.
b. If you can get it done well, then yeah, magnetize the guns.
2. Riptides - 3 of them for the Riptide Wing, rules found in the Mont'ka book set (a deep stab in your wallet for Trip-Tide and Assassin rules, but there ya go). This unit is inclusive on a lot of tourney lists, eldar, tau or muhreens. I've been using it and man, they pack a wallop with HBC guns and durability Riptide durability.
3. A StormSurge or two. Gargantuan Creature rule mechanics are awesome and the Storm's dice output is nuts. Although, I personally haven't spent the money yet.
4. I dunno if I'm sold on Ghostkeels ... I don't own the models (so haven't tried 'em). The few games I have had to fight against them means it is best to ignore them, so far. Their range is medium with a pretty good amount of gun dice. Being highly Shrouded, and being
MCs (an enemy's problem in h2h) means they are a tough unit to kill. At least for how my armies have been set up lately. I regularly use a WraithKnight, and going h2h against 3 Ghostkeels is a mistake ... too many AP2 slaps.
5. Vehicles - Park your tanks. Battle Company is great because
SM get free rhinos (at like 1600 point games in order for it to really work) but the rest of the races have far too high points costs to be competitive on a points-to-transport ratio to be worthwhile. Eldar 6e Wave Serpents had great fire power to make them the foundation of a list, but that's gone now.
My Bob and Fred comic makes comment on that, below.
Seriously, the rest of your purchases are in the Big Boys (monstrous creatures) category, so get ready to spend money.