Hello! I live in Osaka. I'd love to help you plan your trip.
Day 1: Kyoto to Koyasan: Take the Nankai Koya Line to Gokurakubashi Station, then transfer to the cable car to Koyasan. I highly recommend the "Tenku" sightseeing train for the train ride to Gokurakubashi. It's an amazing experience. However, it's a popular train, so it's best to make a reservation in advance.
Tenku Information:
https://www.nankai.co.jp/koya/tenku/Day 2: Koyasan to Kii-Tanabe: As you mentioned, the direct "Sacred Site Pilgrimage Bus" (Ryujin Bus) will only start operating in April, so it won't be available in March.
Ryujin Bus (Reference):
https://www.ryujinbus.com/article/4816/Railway Options:
Via Shin-Imamiya: From Koyasan, return to Shin-Imamiya Station (Osaka) on the Nankai Line, then take the JR Kuroshio Limited Express to Kii-Tanabe.
Via Hashimoto/Wakayama: From Hashimoto, you can take the JR Wakayama Line to Wakayama. However, this is a local line with very limited service, so returning via Shin-Imamiya is more reliable and may be faster.
Local Information for the Nakahechi Route:
Food: We recommend buying "mehari sushi" at the Kumano Kodo Nakahechi Roadside Station. A local specialty, "mehari sushi" (rice balls wrapped in mustard leaves) is perfect for lunch or a snack while hiking. Also, don't miss the "modemochi" (rice cakes) at Kumano Hongu Taisha Shrine, a famous local treat.
Scenery: If you're lucky, you might even be able to see cherry blossoms if you're traveling in late March!
Lodging Recommendation: We highly recommend staying at Kawayu Onsen. This unique spot features hot springs directly from the riverbed. Free bathing in the river (swimsuit required) is available, and there are also indoor facilities available for a fee.
For more information about Kawayu Onsen, please see:
https://setsuyakukaman.com/kawayuonnsenkawaasobi/Enjoy a wonderful trip along the Kumano Kodo!