When to apply for a UK Spouse Visa

Just need a little help as I can't seem to find the answer anywhere online.  I plan to move to the UK at the end of July 2019 with my wife and sons.  What I can't seem to figure out is when I should apply for the spouse visa?  It says it can take up to 3 months to issue the visa which would make the application at the end of April, but how soon after the visa is issued do we need to travel?  Is it worth applying for it earlier so any delays in the application can be covered, or do we need to get the visa and head over to the UK asap?  Also, for the 2 years 9 months validity before we need to extend, does this start from the date the visa was issued or from the date we enter the UK?  I tried to contact FCO.GOV.UK but they want to charge over 5 pounds for an email enquiring about something they should really have up on the site!

Basically if we travel on the 26th July when should we apply?

When does the clock start ticking before we have to apply for the leave to remain extension?

Does it make sense to purchase our flights before applying to prove our travel date?

Appreciate any help that can be given!