There's not much you can do.
They'll offer you some equity, but B2SMB SaaS startups virtually never have a liquidity event. Which means your stock won't be worth anything in practice. I wouldn't trade salary for stock.
They may promise you big raises in the future, but don't expect those to materialize.
I think you should negotiate for flexible working conditions. Creative freedom. Basically anything that improves your quality of life but doesn't cost the startup much.
Mostly though you should be getting a ton of experience of out of this. Then you can do your own startup a few years down the road.