Accept fast offer or wait for better?

I’m applying for finance/accounting jobs in London and I have a UK Dependant Visa. I’m still in Poland right now, but I’m planning to move soon because my partner was relocated there for work.

I have about five years of experience in finance and accounting, mostly in larger international corporations. Currently, I work at a big international tech company.

After about a month of applying, I had two interviews with the same company and got the offer that very same day. I didn’t expect to go through both interviews and get an offer all on the same day. They offered me £36k for the first 6 months (probation period), then up to £40k. £40k was my salary expectation at the time, but now I regret not asking for more. The probation period wasn’t mentioned verbally, just later in the written offer.

The company has less than 250 employees, and it feels like a small, family-run business. The finance team would be just two people - including me and I’d be responsible for the full accounting process. I’m a bit nervous about moving from a corporate environment to something small with a high level of responsibility. Especially considering the relatively low salary.

Now I’m not sure if I should accept or reject and keep waiting for a better opportunity.

What would you do in my place?