I've mentored a lot of young engineers and something I often point out is that there is practically zero risk in accepting an offer. Many companies also do not even present the full paperwork to sign until your first day or two.

You don't have to show up on the start date, though you do have to be mindful of the bridge you may be burning. But it is totally reasonable to show up to a few days of work, and declare "you know what, I don't think this is a good fit" and no one will be offended. They don't have to know that you already accepted another offer.

Exploding offers, low-ball offers, all bullshit. But you don't have to play their games.