SE Asia Last minute hesitations - July/August

Hey all, I am flying out for my first visit to Asia on May 25. I will be spending about a month in Japan before (in theory) heading to Vietnam and perhaps other mainland countries of SE Asia. Initially I had not imagined going during this time of year for heat reasons, but once i found out my friend was working in Tokyo for the summer I thought might as well check out SE Asia since I'll already be flying across the Pacific. I'm having last minute hesitations about weather, especially since I'm pretty active and will want to go hiking, etc. I was most excited about Vietnam (especially the north) and while I've read that people still have good experiences in the country in July/August, it sounds like the north is pretty brutal in terms of weather. I'm also concerned about motorbiking with heavy rain/storm risks. Since I don't have flights or anything else booked, I'm considering other countries during these months. I have been looking into Mongolia, Central Asia perhaps, or even Indonesia since it seems drier during this time and has good diving conditions (I'm certified). Mongolia or Central Asia would presumably be a very different travel experience and I might consider bringing some camping gear and probably more clothing. If anyone has any thoughts about this concern or experiences/recommendations I would love to hear anything! Thank you 😄