In 2020, I neglected to report my IRA contribution from Form 5498. (1) Can I still amend this? (2) To amend, do I just need to file 1040-X?

Yes, if you want to deduct traditional IRA contributions, you can file 2020 1040X to claim a refund.  If you do not deduct, you can file Form 8606 (which can be mailed to the IRS alone) and the amount will not be taxed in the future when you receive a distribution.  See What Is IRS Form 5498? - TurboTax Tax Tips & Videos - Intuit