So first of all,
If your spouse compensates you as a subcontractor, you will pay SECA, but your spouse will deduct the expense from their earnings which reduces their SECA. That's more or less a wash.
Then secondly, you will need to report this subcontractor job on a second Schedule C attached to your return. You generally should not combine two different self-employment jobs on one schedule C unless they are basically the same. (You could combine Uber and DoorDash on one Schedule C as "ride share", but you could not combine Uber and freelance graphic design on the same schedule C.)
Finally, I think you can increase your solo 401k, up to your adjusted compensation from working, even if that work is from more than one schedule C. As you say, by reducing your spouse's net income, your spouse's participation may be reduced. Also, remember the total 401k contribution limit from all sources is still $69,000 (or $76,500) so if you have elective deferrals from a main job in addition to side jobs, that overall limit must be respected. Although I also defer to @dmertz if he is available to give the final word.
