@YusukeUramesh my experience with subs is not the best probably, but if you want to add subtitles to a bluray basically all you need to do is to convert your .ass files to .sup files, you can use easySUP - it works good, also gets the font/positions/size from the.ass files, only problem is that subs with an individual position will be moved a small (but noticeable distance from the original positions, i am not sure if there are any limits for .sup files for positions ... maybe, but i am really not an expert

i haven't tested easySUP that uch but it is very easy to use and i think you may could change or fix the positions somehow (but i never tried so far)