Vanguard wouldn't need to know it was a solo 401k, they would just see a trust being signed up. There are other institutions that you can go into mutual funds or get similar exposure from who are more acclimated to solo 401k entities though.
No, your scenario is not "very easy". No custodian is going to handle a solo 401k based on one dollar of self employed income (and 1099-NECs aren't issued for such tiny amounts anyway). You are also overlooking the overhead of maintaining a 401k, such as plan updates every time related federal tax law changes, or the potential IRS reporting requirements, which can generate significant penalties if overlooked.
OP can also just use the solo 401k's roth balances as a temporary holding place to then pass through directly to the roth ira
The expanded investment options are worth it alone. And the years where there is self employment income, the expanded contribution limit up to doing my own $70,000 employer match saves literal years of contributions off my life.
It's literally 10x what the IRA contribution guys get, and most of them are barely able to even reach the IRA contribution max.
There are more tax deferral plans out there too, I'm willing to do that compliance.
there are caveats to this, like always being attached to your solo 401k plan makes you ineligible for contributions to an IRA all the time, but you will be able to have rollovers into the IRAs, you also might decide that the solo 401k is a superior product to IRAs in every way
if you are not currently eligible to create a solo 401k, it is very easy to become eligible with a single dollar of 1099 or schedule C income the year you make it, and then it can exist in perpetuity
corroborate that with your licensed professionals. many gurus overlook the solo roth 401k mostly due to SEO and their audience of professionals that associate "401k" with "corporate employer thing", as opposed to something at parity with a traditional and roth IRA and expanded in capability