Way to automate a variable irregular IRA investment allocated across 5 mutual funds by percentage?

I currently do this manually. My self-employment income varies widely, so I monthly determine the amount to contribute, transfer it to Schwab IRA, then calculate and place the 5 buy orders. It is tedious and time consuming.

My work 401(k) does that automatically with set percentage allocations.

I would like to be able to set up a saved allocation profile, then on my own schedule:

1. Login to Schwab
2. Set transfer in of $XXXX to IRA
3. Assign that to the saved allocation profile which would calc & place the buy orders

I can't use the fixed-date schedule, fixed-amount, automated investing that I see Schwab has available. Is there some way to do this? Even a workaround using the existing automated investing setup?