Your job classification depends on your relationship with your employer, you can't change it on a whim.
If you have unreimbursed job expenses from your W-2 job, you should discuss reimbursement with your employer. In some states, if you are required to pay certain costs and are not reimbursed, that may be a labor violation. You can't get any tax benefits for unreimbursed expenses from a W-2 job.
For self-employment income (side jobs, gigs, etc.) you report your income and expenses on schedule C, that expense deduction was not changed in tax reform. However, you can't allocate expenses from one job to another -- you can't allocate expenses from your W-2 job to your schedule C, even if they are the same kind of work. For example, you can't deduct mileage from your W-2 job on schedule C, although you can deduct the mileage to and from your side gigs on schedule C.
Remember that each spouse needs a separate schedule C for their own part of their SE income.
Some costs may be difficult to allocate. For example, if you buy a new instrument, can you list it as an asset on your schedule C and depreciate it, or do you only depreciate 50% of the cost, if 50% of your income is W-2 and 50% is self-employment. I'm pretty sure you have to allocate everything, but you would need to speak with a professional accountant for a definitive answer.
