I have always disliked Vanguard's i401k interface, so this Ascensus switch is giving me the impulse to switch my SO's i401k to a different provider. I originally picked Vanguard because we have accounts there, but dislike that I had to make a completely different account for the Small Business side. Account administration tasks, like changing my mailing address, have been painful.
We already have accounts with Fidelity and Schwab, so they are in the running and seem to top the lists of i401k provider ratings.
Another option is to roll the i401k into an IRA (SO has a Vanguard IRA already) and open a new i401k somewhere else. It might be less paperwork overall, but would drop in a big basis in traditional IRA that would impact one's ability to do a Backdoor Roth IRA contribution... if they are near the Roth IRA income limit.