Does experiencing sexual desire in a dream, without seeing any fluid or knowing its form, necessitate performing ghusl after Fajr prayer, especially given that one suffers from severe intrusive thoughts (waswasa)?

Ghusl is only obligatory upon certainty of seminal discharge. If a virgin woman discharges semen into her vagina, ghusl is not obligatory upon her until the semen exits her vagina, because the inside of her vagina is considered an internal part. As for a non-virgin, ghusl becomes obligatory when semen reaches the inner part of her vagina that becomes visible when she squats to relieve herself. Mere wet dreams or feeling aroused do not necessitate ghusl unless you are certain of seminal discharge.