Kings Langley is another station that's very close to the M25, and would avoid the trek into Watford for northbound travellers.
Unfortunately, Kings Langley only gets 4 trains per hour (two up , two down), and after about 7am all the north-bounds terminate at Tring.
Not that I've got anything against Tring - I prefer to change there rather than at Hemel Hempstead on the way home in the evening.