Yeah, I think the issue is telling a client with a straight face you're going to add a "not quite ready junior dev" to a project to QA your own work for the cli

Yeah, I think the issue is telling a client with a straight face you're going to add a "not quite ready junior dev" to a project to QA your own work for the client and charge the client for the privilege (even at a "below market rate") is a bit ... off. And if you're doing it fixed-fee, then you really need to confirm that the QA is either accelerating delivery (and therefore reducing costs) so you can keep the price constant and collect more margin or ensuring a better (i.e. more valuable) outcome to the client to justify an increase in price in a fairly competitive bidding market.