Am really sorry to learn you're dealing with cancer. I'm six months post-chemotherapy, so understand what you're dealing with, particularly the hair loss. It's really hard. I refused to take steroids which meant I didn't gain terribly much weight, thankfully. Even so, I know how hard this can be as a friend of mine gained masses during treatment.
Have you spoken to Macmillan yet? They will give you a grant of £350 which can be spent how you like. Many spend it on travel to hospital, others spend it on clothes, head scarves etc. Also, Maggie's are helpful with workshops regarding how to tie head scarves and what to wear. Be sure to contact them too.
In terms of clothing, natural fibres are you friend. Layers too. Chemotherapy can caused not flushes and night sweats. Look on Vinted and double check the fabric content. As you're having surgery on your neck, lots of front fastening shirts so you don't need to lift your hands. If you can, take a hospital gown home as they're ideal for wearing round the house. I wore Crocs and Birkenstock clogs with socks, both things I never would have worn prior to cancer but I had terrible peripheral neuropathy in my fingers and could tie laces. Also, I believe Sketchers can be slipped on without having to tie laces. With regards to bras, I wore a front-fastening one, thankfully provided due to the cancer I had, and also Sloggi-like crop tops as they were easy to pull on/off.
Hair. I had mine shaved to a grade 2 after the first round of chemo as I was shedding hair everywhere. The inbetween stage - super short to bald was far harder to deal with, but once most of my hair was gone, it was a relief as I found my hair falling out super painful. Macmillan can help with a wig (free of charge). I didn't wear mine. I actually preferred a bare head as it was so much cooler than wearing scarves/hats/a wig!
Lastly, and if possible, look after yourself and treat yourself compassionately. If you're able to, get dressed, put make-up on, more so when you're going for treatment as it will make you feel so much better.
Wishing you well.