Your 3/4/5 were sort of bad potentially because you were quite likely to split you 1/2 picks with the opponent resulting in you getting two of the 3-5 cluster. If you got 3 and 4, or 4 and 5 that was fine, but getting 3 and 5 could have been very bad for you.
If I was going to pick SEA and EC as 1/2 though I'd reverse the order, the double border EC has on SEA typically makes it the better territory, the main exception being when EC is a 3 turn bonus and SEA isn't.
grabbing Jiangxi turn 4 was a good move, letting him grab the double border while taking SEA would have surely wasted your armies spent taking SEA.
I'm not sure I'd have hit Iran turn 5. Xinjiang, and Tadschikistan are both arguably better choices. Xinjiang for protecting SEA, and immediately bordering CR, Tadschikistan for being the fastest route to Eastern Kazakhstan which is even more dominant over the Russias than Georgia is.
You seem to struggle with knowing when a game is won. If you'd never entered Africa and just used your 2 income lead and superior positioning to threaten Russia you had this game won very easily very early, going for more income, and as such threatening his position less effectively is what let the game drag on for so long.
Later you could have won the game faster as well by reaching Xinjiang earlier to threaten CR, and Cauc.
I have no clue why he decided to go for ER over finishing Europe or taking Scandi, both seem better choices to me, WC threatens ER too much for ER to be remotely safe.
Also, in turn 7 it would have been gg if you had hit Eastern Kazakhstan with everything, as you'd have had three borders on his two main bonuses, while CA/SA were completely safe and SEA was sufficiently defended in Hong Kong.