You seems to have misunderstood the concept of na-adjective. All na-adjectives accept -だ, so just because you can say 強烈だ does not mean 強烈 also works as a simple noun. A simple noun can take case particles like は/が/を, but you cannot say 強烈がある or 強烈を見る, right?
- 元気: a na-adjective that also works as a standalone noun.
元気な人。彼は元気だ。元気がある。元気を出す。
- 良質: a na-/no-adjective that can only describe a noun (attributively or predicatively). It very rarely works as a standalone noun.
良質なダイヤ。良質のダイヤ。このダイヤは良質だ。(×良質がある。)
- 強烈: a na-adjective that can describe a noun (attributively or predicatively) or a verb or another adjective (adverbially). It cannot work as a standalone noun.
強烈な台風。この台風は強烈だ。彼を強烈に殴る。これは強烈に辛い。(×強烈がある。)
So quite contrary to your statements, 良質 and 強烈 are "purer" na-adjectives, and your question seems baseless to me. If a word works as a na-adjective, you can attach そうな to it, at least grammatically. But it does not mean na-adj + そうな must always make sense. 良質そうな車 and 強烈そうな台風 are unnatural for different reasons.
- 良質そうな車 is unnatural because 良質 usually describes natural products like fruit, ore, oil, etc. But you can safely say 高級そうな車 instead although 高級 is a two-kanji kango word.
- 強烈そうな台風 is almost always unnatural because そう always involves a direct observation, and ordinary people cannot directly observe a typhoon and say "That typhoon appears to be furious" before knowing about it on TV or the internet. If you were an astronaut and noticed a large typhoon on the earth, you could safely say 強烈そうな台風が見えます.
And statistically speaking, I think そう tends to be used with i-adjectives more often, because observable simple concepts are covered by native Japanese vocabulary (wago). Kango words are better at describing abstract concepts, but they are unlikely to be used with そう anyway.
