The single-sheet road maps of the whole of Taiwan are OK for major roads, and they have some Romanization of place names as well.
For more detail the 1:100,000 single-volume maps are generally OK although they don’t have Romanization – don’t know how your Chinese place name recognition skills are. For a little more on map reading see here:
[Forumosa - Taiwan's largest and most active Taiwan-oriented global online community in English … 860#138860](Pinyin and Characters of Same Map for Taipei and Taiwan? - #5 by Joesox
and here:
[Forumosa - Taiwan's largest and most active Taiwan-oriented global online community in English … 248#145248](Motorcycle touring - #129 by Joesox
The Gaoxiong route is a difficult one to decide. You’re going from Taipei, right? The Provincial Highway no.3 is a good choice for Taipei-Taichung, but after that you could continue on the no.3 most of the way to Tainan (interesting scenery later on but takes a long time and plenty of traffic lights in the first section) or the Provincial no.1 to Tainan (much quicker but no good scenery). I would say that the no.3 would take a good 2 or 3 hours longer than the no.1. Probably the most scenic part of the no.3 would be the reservoir (Zhengwen or Zhongwen: I’m not sure which) and its surroundings. I say probably because by the time I reached it night had fallen and I couldn’t see anything.