Mailed amended return 5 months ago and IRS still not showing it as received. Sent via certified mail. Should I re-file electronically now that I’m able to?

The IRS says to not file a second time as that could delay things further.  Paper returns are slow to process especially if you filed before the tax law changes or have a stimulus credit on line 30   so  if you are sure the IRS did get the return then wait you must.

Do not file another amended tax return.  It would only prolong the process even further.

As stated on the IRS website for the status of an amended tax return - https://www.irs.gov/filing/wheres-my-amended-return

COVID-19 Mail Processing Delays

It’s taking us longer to process mailed documents including:

• Paper tax returns, and
• All tax return related correspondence.

We are processing all mail in the order we received it.

Do not file a second tax return or call the IRS.