If you have already been to Sofia, maybe you could skip it and pass directly to Plovdiv and maybe as far as the Bulgarian black sea coast? Check this post from @valensia:
https://steemit.com/travel/@valensia/have-you-ever-been-to-bulgaria-no-now-is-the-moment
it has everything you need about Bulgaria :-)
Thanks, about to read that now. I think I would skip Sofia, probably just do an overnight there as a stopover.