Everywhere is safe in Himachal to travel solo😊 however I'll suggest you to go through an Instagram page named mountainbytes786 ; they organise trekking expeditions around himachal and uttarakhand and take you to the places you have never explored😊
I recently travelled in Himachal for around 15 days and it was super safe. Kasol i really loved and probably it can be on top of your stay list being a solo traveller. You will find lot of people doing work from mountains here right now so overall like minded people :) Also, it has got lot of near by treks to go for and are very beautiful
Reason i liked Kasol is you can explore lot of near by treks while being there. Kheerganga, Grahan Village, Challal, Malana etc are few beautiful treks i went to while i was there
Https://www.tripoto.com/himachal-pradesh/trips/7km-from-manali-head-to-this-mystic-homestay-for-the-perfect-escape-from-busy-life-5fa6be60b3bc5 and https://www.tripoto.com/trip/feel-the-magic-of-autumn-in-sissu-spitis-best-kept-secret-5fa979a9a434b
I stayed in Kasol for around 12 days. Check the few details here: https://www.tripoto.com/himachal-pradesh/trips/want-to-work-from-mountains-head-to-kasol-for-a-perfect-workcation-5f906b0e5b4f7. After that i stayed in Manali and visited Spiti as well. You can read these for more details. Adding the links