How much money do I need in my bank account if I am planning to stay for 4 months?
For this specific question, please see this other question which could help:
You need enough money to show the officer you have enough money for accommodation, food and travel so that you won’t have to work illegally while in the UK. They also want to see a steady income each month on your bank statement.
Could I use a train ticket to France or plane ticket to France rather than a return ticket to Taiwan?
See this other post:
No. UK does not require proof of return/onward travel as a matter of routine and if you don't have such a ticket you can't produce it on request at a border.
However, it is not very likely that you will be asked for a return ticket unless there is some doubt about your returning and then having a return ticket should help
Therefore, a return ticket necessarily isn’t required, but if asked for one, you could show a ticket to France which means you are planning to leave the country.
My real plan is to be a nomad in the UK, but i dont want to complicate things for the officer in the airport, could I just simplify my schedule (pretend to be a one month visitor in the UK?
No, tell the officer the truth. If you lie and they find out, you could face a ban and serious immigration problems.
How could I prepare the accommodation proof if i am going to use couchsurfing for 80 percent of the time while staying in the UK?
I recommend presenting proof for a place you will be staying a long time, e.g. a few weeks, or say you are couch surfing and present multiple proofs of accommodation.
Although Willeke did mention in a comment that a young person, planning to travel around, it would be acceptable to have the first night in a hostel or cheap hotel and a list of hostels or some such, to book after that. But if you do so, make sure you have a budget that allows you to travel that kind of hotel or hostel.
And know you can not rely on finding couch surfing hosts, certainly not in this (still or just post) lock-down situation.