spent half the last decade in seoul, my impression is that it's both far easier and far harder to fully feel like part of the group in korea as a non-korean. the first hurdle, which most foreigners (including me) never fully make it past, is the language. after that, it is down to the specific people that make up the group around the foreigner, and i think this is true of any humans anywhere. my impression was that the most cruel forms of exclusion/discrimination in korean society were reserved for other east asians/asians who speak korean as a fluent L2. this will probably come to a head, hopefully with positive results, in the next 15-20 years given that there is a large (non-trivial) subgroup of children in korea now, mostly outside seoul, with a parent (usually the mother) from a south east asian country.
on the topic that kicked off this subthread (i think--about stubbornly non-adaptive western foreigners in very different cultures than their own), when i was doing ~25 hours a week of korean language classes and study in addition to a full time job, i used to tell that particular subtype of Complaining Expat that if they wanted to level up their complaining about the host culture, they should learn intermediate korean, if only for the sole purpose of unlocking a whole new ocean of complaining material. this was obviously bait/a joke, but there are no jokes: it's one thing to have someone make basic hand gestures like "eating" etc to you when they know you absolutely don't really know what you are saying; it's another thing to have someone do it when you have been conversing with them in their language for the last five to ten minutes.
i could do like fifty posts about race and foreignness in korea but i always think of the model who has appeared in a lot of korean shoe and clothing ads recently. his parents are nigerian and korean and he grew up in korea with korean as his first language, and in an interview he once expressed goodhumoured frustration about ethnic-korean people speaking English to him by default when his english is, by his own admission, not that great at all.