The mark only appears once the marked one appears. The marked one is Tanjiro but his mark did not appear in the Mugen Train Arc or before, it appeared in the Swordsmith Village Arc. It is also mentioned in the season 4/Hashira Training Arc that for a mark to appear the marked one has to activate the mark first and then only anyone else can activate their mark. Muichiro and Mitsuri's mark appeared cuz Tanjiro activated his mark in that arc first. (Sorry if i said anything wrong. Feel free to correct me)

demon slayas

But when Akaza fought giyu, he said quote “I haven’t seen someone this good in nearly 50 years”

My theory is that Rengoku did actually awaken his mark and the transparent world

The mark appears once the sun breathing style demon slayer mark is awakened. Which tanjiro didn't awaken until the next arc.

he did awaken his mark and he unlocked Transparent world too.

He can't because the marked one hasn't unlocked his yet which is tanjiro

The mark did not appear cuz there's not enough backstory shown 🤣

weak writing