豊かとはちょっと違う気もするけど
も in the above phrase is a particle that means also; in addition; as well. I think it is exactly what is said in your quotation: according to this question's answer, it appears that using も, as opposed to が, expresses "multiple opinions."
I'll show you easy examples as:
- この果物｛くだもの｝の味｛あじ｝は、甘｛あま｝い気｛き｝がする The taste of the fruit is sweet.
- この果物｛くだもの｝の味｛あじ｝は、甘｛あま｝い気｛き｝もする The taste of the fruit is not only sweet, but also ...
When the taste of the fruit is sweet, you could express the taste definitely by phrase 1.
On the other hand, when the taste of the fruit is almost sweet but a little sour, you can't use phrase 1, so you would say by using も like phrase 2.
In this case, phrase 2 implies この果物｛くだもの｝の味｛あじ｝は、甘｛あま｝い気｛き｝もする、 そして、少｛すこ｝し酸｛す｝っぱい気｛き｝もする。
As the conclusion, the given phrase 豊かとはちょっと違う気もするけど implies that speaker can not definitely say only 豊か richness by using も instead of が.
Another reason to use "も" instead of "が"
I am going to talk about another reason why Japanese people use the expression "気もする" that is used in the given phrase with "豊かとはちょっと違う気もするけど" more frequently than the theoretical reasons.
I mentioned above that "甘い気もする" is not an assertive expression than "甘い気がする". Naturally, if you are fully confident in what you are going to say, namely if you feel the taste of the fruit is sweet without doubt, you would say assertively "甘いと思う" or "甘いです". If you say something in this way, I think it is common in the world that people who do not think so will object to it.
A: "I think like this."
B: "No, I don't think so."
However, in Japan, although what is right (/correct) and what is wrong (/incorrect) is naturally important, the sense of value of "wa 和" or "harmony" is sometimes more important than to assert the rightness or correctness in order to avoid conflict among the people in the society.
Although this value is said decreasing these days in comparison with the old days, it still seems to be common among Japanese people.
"[和]{wa}" seems like a good ingenuity that the predecessor invented in order to avoid such a confrontation.
Therefore, Japanese people sometimes use expressions such as "気もする" even if their true intention is to say "気がする", so it is annoying but prudent to know that there is a case where speaker avoids conflict with the opponent by saying so.
