Help please!!! Tier 5 - want to extend my stay < 6 months

Hi everyone,

I need some help please! I am currently 1 year into my 2 year YMS visa. I came to the UK with my boyfriend who has a UK passport (but had never lived in the UK before now), and we have now been living together for 1 year. My Tier 5 YMS expires in August 2019, but we would like to stay until around December 2019 (for reasons related to my boyfriend's work and study). I am currently only working temporary jobs so Tier 2 is not an option for me unfortunately.

I have read about the < 6 month marriage visa for fiances? As well as applying for a spousal visa of course.

With the switching over to a Spousal visa, is getting a marriage certificate enough, or will we have to hold an actual ceremony in a licensed venue?

Are there any other options? And given we are only extending our stay by about 4 months post-Tier 5 expiry, we would like to keep the cost low if possible.

Would really appreciate any advice, thank you in advance!

Kind regards,
IK