OP please find a new job, this employer is very poor. I have a background in managing SME businesses and what you are describing is exceptionally bad even for post-covid times.
No good employer would set up their business so that all the workers come in together early, take breaks together, socialise and leave mid way through the afternoon EXCEPT one very new/junior person who is expected to work alone for 90 minutes (hell not just 90 minutes but basically all day as their timimgs don't align with everyone else).
It also sounds like the managers don't manage or appear in the office either.
The lone working is poor because it is totally unnecessary, I would expect at least 2 people on site at all times and especially when locking up, there are not words to express how angry I feel on your behalf about you being told you are responsible if anything is taken/happens when you lock up.
Even when I was the MD, I hated locking up at night in the winter when it was dark. It was a very rare event as although I was often one of the last to leave, when it was time for the later workers to finish we all left together. Checking that all the cars started, giving lifts to town or whatever was needed.
The 3 months notice of leave and not being allowed any time off at Christmas when everyone else is just reinforces how bad they are.
Will you find it easy to get a better job?
I would walk now, and if this was my friend I would tell them to do the same, but have no idea of your financial situation.