In this answer Rickross provided exemption for Sandhya Vandana during ashoucha period.
Sandhyopaasana needs to be performed both during ‘Jaataashoucha’or ‘Mritaashoucha’ or Ashuddhi on account of births or deaths, by maanaso cchaarana or recitation of mantras mentally without resorting to pranaayama, confirms Pulastya Maharshi.
Question is, which steps in Sandhya Vandana are allowed and which ones not? Like cleansing/aachamana, pranayama, sankalpa, arghya, guru Vandana, dik-vandana, nyasa, dhyana, japa (only Gayatri allowed or others like astakshari too)? Which of these should be Manasa? Read somewhere that one should do without Kusha and water.
If not from full references, I am fine to accept the practice/experience as an answer here.