Is it necessary to change clothes for prayer when there is a suspicion of madhy (pre-seminal fluid) discharge after returning home, or is it permissible to pray in ritually impure clothes if this occurs frequently and it is difficult to change them during travel or outings?

The fluid you find, which is normal vaginal discharge, is not madhiy (pre-seminal fluid). It is pure (tahir) and does not necessitate istinja' (washing the private parts) or changing clothes, but it does nullify wudu' (ablution).

If you feel it exiting, perform wudu'. If you find it on your clothes after prayer and it is probable that it exited after the prayer, then your prayer is valid. If it is continuous, then your ruling is that of one afflicted with salasil (incontinence): you perform wudu' after the entry of the prayer time and pray the obligatory and supererogatory prayers, and do not delay the prayer. If you are in a state of wudu', then pray. If you doubt its exit, the default is that it has not exited. If you are certain of its exit, then repeat your wudu' and pray.