There’s quite a bit of research on lump sum investing vs dollar cost averaging. Here is one example: https://investor.vanguard.com/investor-resources-education/news/lump-sum-investing-versus-cost-averaging-which-is-better
Generally lump sum investing comes out ahead by a bit. However, my personal opinion is that it isn’t enough to always point to it and say that’s what you should do. If you’re more comfortable doing one over the other, then do it.
Generally time in the market beats timing the market, which is what you’d be doing by dollar cost averaging because you think the market is going to crash.
You won’t be making a mistake by picking Fidelity. Vanguard is also a good choice as others have said. Fidelity generally has a better UI than Vanguard if that is important to you.