Help RESIGNING! so that I can come back! BC

So I left my job 2 months ago, the work culture and CTO was very very toxic, couldn’t handle it. So I get offer from a startup, nice start up, good work culture, good learning. I joined this startup 15 days back. Now I got an offer from a established mid size company whose pay is 35-40% higher, it’s a substantial amount so I signed their offer letter. Now I wanna leave my current startup but I dont wanna burn bridges while leaving, I have a doubt that if things dont go well with the new offer, cause I left without serving notice period from the toxic one, so I have this paranoia that something might go wrong with the onboarding for the new offer. I dont wanna leave such that my current startup doesn’t even consider me for rejoining. I want a solid excuse for resigning, such that even if after 1 week I want to return to A, they would consider me! I am saying this because I worked my ass off for 3 months to get a job and I got a good start up, nowadays it’s hard to find decent workplace, don’t wanna end up with nothing if something goes wrong with the new one! HELP!! I was thinking about some family emergency so can’t continue, Any other solid reasons???