So you are not "only getting royalty payments in retirement." What you call occasional side jobs is an active business with a significant amount of income. You have to continue to report it as business income on Schedule C. You probably don't have any expenses to collect the royalties, but you probably do have expenses for the side jobs, and of course you can deduct those expenses on Schedule C. Include the royalties in the income on Schedule C.
You can continue to contribute to a SEP as long as you have net self-employment income. Note that if you are 73 or older you must take RMDs from your SEP IRA.
