For travel date, you want to put the earliest possible date you will want to travel to the UK. I'd work backwards from when you want to get married.
After you arrive in the UK, you'll need to have your name put onto something to receive an item in the mail to register your notice to marry. You'll need to have been in the UK for a minimum of 7 days to do this. I'd allow a couple of weeks to physically get something.
Once you have something, you'll set up your appointment to give notice at the registry office. Sometimes it can take a few weeks to get an appointment, as foreigners can only give notice at certain registry offices.
Then you'll need 70 days prior to the wedding date. While you *should* have your clearance from the Home Office back to the registry office in only 28 days, the registry office won't commit to earlier than 70 days.
So I'd work backwards from the date in December.
So, say you are looking at Saturday, December 2nd. 70 days prior is September 23rd. Hopefully you could book an appointment for then at the start of September. And if you allow yourself a few weeks to get your name on an item of mail, I'd aim to move over early August.
You can apply for the visa up to 90 days before you want to travel - but you ARE at the mercy of UKVI issuing the visa for the correct travel date. It's just plain hard to marry in the UK, which is why so many of us chose not too. If you apply in May and request a travel date in August, there is a chance the visa could be issued in May or June.... with an immediate travel date (that would be naughty on their part, but it does happen). Then you would have to be married and have your FLR(M) application submitted by November/December. At the same time, you could apply in May and request a travel date of August and get caught in the summer backlog and your visa might not be issued until September/October/November, meaning you wouldn't have time to give notice and meet the December wedding date. Just be sure to keep things flexible until you have your visa in your hand and know for sure when you'll be able to marry.
Yes, you can schedule an in-person appointment for FLR(M) prior to the marriage, so long as the appointment is for AFTER you are married. You just want to be 100% sure you'll have everything you need on the day of the appointment, so I'd allow a week or two buffer just so you aren't stressed about the appointment at the same time as the wedding.
Don't forget that you'll need medical insurance while on a fiancé visa!