Hey, fresh startup here. We use Gusto and selected Guideline out of the two options presented, mostly because we have a ton of things we're focused on. Always had a feeling it was a scam, but didn't have time to really dig into it (we're also pretty small at the moment)

What's the best way to transition to Fidelity / Vanguard? I assume Fidelity would be better for having a single entity to deal with rather than Fidelity for HSA and Vanguard for 401K?

Call Fidelity and ask them - they'll happily walk you through it!