Is it permissible to directly address Allah (swt) in the Tashahhud during my prayers in this way: “الحمد لله أشهد أن لا إله إلا أنت وحدك لا شريك لك وأشهد أن محمدا عبدك ورسولك”?
No. We must pray as the Prophet Muhammad (SAWA) taught us to pray (Pray as you saw me orating صَلّوا كَما رأيتموني أُصَلّي
He was saying Ashhadu ANLA Ilaaha Illallah WAHDAHU Laa Shareeka LAH Wa Ashhadu Anna MUHAMMADAN Abdoho Wa Rasooloh, ALLAHUMMA SALLI Ala Muhammadin Wa AALI Muhammad)
Wassalam.