Me and my wife went through what you had asked above and i guess i can share some insight from our experience.

You had answered most of the questions yourself but here is some summary you can refer to :

Most important thing is visit UTC Pudu as all the procedure can be done in the same building and you can proceed to submit the form to Thean Hou Temple which is very near.

1. Go JPN request for ROM form
2. Fill up and sign it infront of a Witness such as Lawyer which you can found in UTC also.
3. Submit back to JPN and pay RM20 and inform them that you will want to perform the ROM on Thean Hou Temple
4. JPN will return you the ROM form and you may proceed to take the form to Thean Hou Temple
5. Go Thean Hou Temple counter and inform them your expected ROM Date and they will check for availability.
6. Once confirmed, you have to pay RM200.
7. On ROM Day itself, bring along your family / photographer and enjoy

On your questions :
1. What other steps am I missing out? Will need to know cause need to plan take IC photo and all for groom and I

Need a normal passport photo of your husband and you to attach on the document

2. Thean Hou Temple will I need to line up on the day itself? At what point do I get my number or date

You don't have to line up, the payment you made for RM200 is a booking for specific time slot example 15-March-2020 @ 10am.

Thus you and your groom family need to be there by 10am.

3. I also read that having a ceremony there, must follow the customs. Can I know what customs? This is because I am Christian and my groom is buddhist.

Because both of you are in different religion, you need to choose either one of the customs as the official from Thean Hou Temple will only able to cater either Christian or Buddhist customs during the official ROM day in the temple.

4. True that I cannot do anything now? Thean Hou gong said to only apply end july.

Yes, if your expected ROM date is September, then you both shall plan a trip to Thean Hou Temple on end of July or early August to book.

This post has been edited by Garenyu: Mar 11 2020, 10:29 AM

You can bring a few person even Guess during the solemnization.

Witness is just use for documentation purpose.

Myself had bring in 2 photographer which is non related to us too...

But please do double check with Thean Hou Temple Admin if they have any limitation due to Covid now.