Leave it as it is. like you state, you are limited on how far into the frame you can go. if you make it too short, a taller rider wond be able to get a post in far enough and will still have issues.
The trend in the bike industry is to have a longer dropper post with a shorter seat tube. For example the ibis Ripley and Ripmo, two of the best selling bikes in the industry, have ST lengths of 418 and 432mm respectively. It allows for more customization via different length dropper post and is much nicer as you can move the seat way out of the way when you are descending. With dropper post reaching lengths of 240mm, it shouldn’t be a problem accommodating longer legged riders. Additionally, droppers are starting to have shorter and shorter insertion lengths, which can make up somewhat for a short frame insertion length. If this seller wants to sell the most amount of frames, he should try to lower the seat tube length. As it is, it will deter buyers looking for a frame with modern geometry.
