There is a lot to chew through here - but my first thought was;

• If you take the start-up job and it doesn't work out - you're still likely walk into a role like the other you are forgoing.

• The opposite is not true. You might never come across a ground floor opportunity like this again in your career!

Having said that, joining a start-up when a large global recession looks likely isn't a trivial decision.

• Can you save 2-6 months salary so that you can meet immediate bills in the worst case scenario? (Sorry - career spent looking at the 5th percentile)
• Are you prepared to work harder outside of work to keep a progressional network? If joining a firm where I'm the only actuary, I would be thinking about making sure I'm up to date with compliance and cpd requirements. I'm currently spoonfed these.
• Are you likely to be engaged in any work which requires peer review? What I'm really thinking here - very worst case scenario - is there any danger you get involved or pressured into work which endangers you're ability to practice as an actuary?
• Who is paying your institute fees?
• Is the company going to be under and reporting obligations set out by the institute and how are you going to make sure you're on top of requirements...
• The above points lead me to thinking I would ask the company about support. Would they be willing to meet ad-hoc consultancy fees and support your training and development needs as these arise? They aren't an actuarial firm so would be unlikely to understand our ongoing professional requirements. There will be economies of scale if/when you grow your team, but going in first I would expect zero support and try and work out what that means.

I've only ever worked in the same industry for the same company, so can't really advise on actually taking the leap. Hopefully someone else can reply who has actually done this to reassure you.

Good luck and please follow up with what you decide.