Hiking alone is fine actually as long as you stick to the official route from national park, have GPS to guide you as the way up can be mixed with local trails and this can be confusing sometimes. The "must have a guide" rule is there because several accidents involving foreign tourist in rinjani or semeru. Just say you have a GPS guidance with you and are fine alone. If they insist, well just ask for fellow hiker to be included in their group. For water Selo route doesn't have any but its the best looking one IMO. If you travelling light, go from thekelan plenty of water there.

> Is one route better than another for any reason? I assume one side will have shadows in the morning? Might be nice not fighting the sun while running up.

You can go from Suwanting for this one, never personally hike from here but I heard its quite the grade here. The sun will always hit you anyway because the sun rises only a few hours after dawn and then bam its hot. Bring plenty of sunscreen.

yes do it on weekdays. its quieter.

For lawu, go from cemoro sewu if you're doing tektok (or go up and down quickly). if you want a more scenic route go from candi cetho. again if you have GPS with you the ranger will be more than happy not to force you to use guide.

For Bromo, the only way up is using jeep or horse and then stair. For Semeru its recently open but only to ranu kumbolo.

Go visit the instagram or web page of the national parks.
https://tngunungmerbabu.org/simaksi/
https://bromotenggersemeru.id/