With that bulk and 3 wonderful recovery options I'm surprised nobody has mentioned Assault Vest.
Making him so much tankier you can run bulk up set with horn leech or milk drink for recovery, or both honestly.
A sub seed set with those boosted defenses? This is one sturdy goat, especially with an Assault Vest.
Assault Vest is still an option, but you must run Horn Leech for recovery. Actually it still sounds totally fine. With 123/81 special bulk you are slightly bulkier than Full HP/Full SpD Positive Celebi without Vest, which is something great to see. With Assault Vest your special defense reaches 430, while base HP 450. If we look for something with similar special bulk, we need to look for Max HP/Max SpD Ho-Oh, which reaches 417 HP and 447 SpD when fully invested. So yep, your special bulk is even slightly... higher thanks to higher base HP. While Grass typing is far worse then Fire/Flying when it goes to resistances, with that special bulk you should be able to tank some SE hits as well easily. While Assault Vest TTar gives you heavy competition here, you have different type of resistances so you have easier time handling for example water types and you actually can handle those Focus blasts more then once, so it's cool. Also Leaf Storms are not a problem for you. And 100 base attack is good enough to hurt stuff back. So in other words I would go with...
Gogoat @ Assault Vest
Sassy Nature
252 HP / 252 SpD / 4 Def (EVs are possible to change)
~ Horn Leech
~ Earthquake
~ Payback
~ Hidden Power Fire / Hidden Power Ice
Technically this looks like a special tank. Tank hit and hit one back. Horn Leech is your recovery move, which should allow you to stay in longer. Although I would still recommend Wish Support for this guy, it should be fine as defensive pivot for bulky offense or some other things. Some random calcs (I included nerf/buffs to moves). I count Assault Vest like +1 Boost (similar to Light Screen).
Max SpA Modest Nature Specs Heatran Fire Blast vs Max HP/Max SpD Positive Nature Assault Vest Gogoat
72.00%-85.33%
With Timid:
65.78%-77.78%
Offensive Max SpA Fire Blast Timid Heatran
44.44%-52.44%
Max SpA Mild Nature Life Orb Draco Meteor Hydreigon
36.00%-42.67%
This time Fire Blast
40.89%-48.44%
Modest Choice Specs Draco Meteor
41.33%-48.89%
Choice Specs Modest HP Fire Magnezone
26.67%-31.56%
Choice Specs Modest Thunderbolt
14.67%-17.33%
Choice Specs Modest Water Spout Jellicent
18.44%-21.78%
Choice Specs Modest Ice Beam Jellicent
29.78%-35.11%
Choice Specs Timid Draco Meteor Latios
38.89%-46.00%
Choice Specs Timid Hidden Power Fire Latios
24.44%-28.89%
+1 Life Orb Timid Dragon Pulse Latios
33.33%-39.33%
Modest Choice Specs Adaptability Dragalge Draco Meteor
46.67%-55.11% (Unfortunately the same problem as with Fairies - You don't like Poison Adaptability STAB)
Modest Choice Specs Adaptability Dragalge Sludge Wave
68.44%-80.89%
Why not some Uber calcs for fun.
Choice Specs Modest Full HP Water Spout Kyogre Under Rain
40.89%-48.22%
(As Long SR is not on field, you avoid 2HKO)
Choice Specs Modest Ice Beam Kyogre
44.00%-52.00% (Risky, but without SR 2% chance to 2HKO back)
Choice Specs Modest Surf Under Rain Kyogre
24.44%-28.89%
Choice Specs Modest Thunder Kyogre
20.00%-23.56%
Max SpA Lustrous Orb Hasty Surf Palkia Under Rain
18.00%-21.33%
Max SpA Lustrous Orb Hasty Spacial Rend Palkia
33.33%-39.33%
Choice Specs Timid Palkia Draco Meteor
43.33%-50.89%
Choice Specs Timid Palkia Spacial Rend
33.33%-39.33%
Modest Choice Specs Kyurem-W Draco Meteor
47.56%-56.22%
Well, I checked each calc two times just to be sure, as they look impressive. Just in case - someone check those out if I didn't made any mistakes. I don't think I did though, but please, as in some cases I still can't believe and in some of them my jaw dropped down... 123/81 bulk with assault vest is just good. As I said - even without Assault Vest special bulk is comparable to SpD Jirachi or Celebi. With it... well, yeah.