I played both as Sukhothai and Khmer, though in 1.3... so this is a bit speculative. I love them both; they're shades of once mighty empires and the upstart Ayutthaya must go down.
Khmer isn't that bad. It's relatively strong. Ayutthaya is likely to enter alliance with Dai Viet, and don't declare on Day 1. Pile some gold and wait for everyone to get their alliances going - which sounds counterintuitive, but Lan Xang and Champa are likely to se DV as rival and deny the access, whereas if you declare on Day 1 then they will get called in nonetheless and march down. If anyone supports your independence, so much the better but hopefully Lan Xang will stay the hell out of the equation. You don't want them to support you and get involved, otherwise DV will march through their land. (Rival them; you can't be friends with them anyways, since you have a core there.) Ayutthaya is likely to send a gift at least once, so mercpile on them and hope for the best, and take your core Khorat in the mean time.
Sukhothai is a bit more luck-based. The personal union is impossible to break forcefully, though there might be a way now due to the support independence feature. I played Sukhothai twice, and the PU broke before 1500 both times naturally as Ayutthaya is kind of surrounded by enemies. (It's the local small-scale equivalent of France) The problem is the PU timer is set to begin in 1438, so Ayutthaya can click the integration button on 1488, which might require more than a bit of luck to make it dissolve naturally. Shan was an important ally in both campaigns, and Zhou in the second one when they revolted out of Ming. The interesting thing about Sukhothai was that the map would change completely by the time you're independent, and everything will need to be figured out as you watch the game unfolding itself while waiting for Ayutthaya to lose the PU...
That said, if OP is OK with Albania and Chimu, playing as a vassal or junior partner isn't going to make a lot of differences. My suggestion for his next challenge would be Ashanti - you have a gold mine there, but your only neighbor is Mali and you're animist.