Depends on the product what you actually need, but I've personally watched more than one successfull software product launched with a more or less this resourcing. Not exactly this recipe but very much in the ballpark.
The assistants aren't assistants - you hire them with minimum pressure, but try still to find the most talented junior you can find.
I'm sure you can be elastic with the mids depending on how much AWS there is to configure or some-such.
2 juniors in a startup is a bit, probably not the right thing. You'll run out of junior work surely?
The "two good ones plus exactly one junior to make them feel important, and to keep the show on the road for 2 weeks in case they both quit simultaneously" formula is pretty solid.
To be specific, my experience was from non-web software.
I would consider "junior" here not as a person in non-critical role who cannot function as an individual contributor, but as a solid candidate that lacks industry experience but can be at the moment hiring be expected to quite fast grow into the role of a solid IC. I.e to use a bit too stereotypical characterization
- not a "stablehand" but a "journeyman apprentice".
At the moment of hiring you don't count on it that they can deliver (but guess and hope), whereas the seniors will have shipped products under their belt.
Mid = journeyman, "worked another job before". Independent, trusted to be delegated to for specific things, but doesn't quite have a personal brand.
Senior = Master. Proven track record, proven reputation of delivering, capable of leadership inside a technical project. Has and maintains a reputation as someone you can trust.
Principal = Mentor to seniors.
Ofc there are shades of grey between junior and mid, because junior-ship takes like 2-3 years to grow out of.
The list of work able to be delegated to someone at the start is not all that large in many cases.
The assistants aren't assistants - you hire them with minimum pressure, but try still to find the most talented junior you can find.