I'm going to be that person. It's President pro tempore . The last non-president to lie in the Rotunda was Rosa Parks, who lay in honor in 2005. Before her, Jacob Chestnut and John Gibson, two Capitol Police officers killed in the line of duty, lay in honor in 1998. In 1989, Claude Pepper was the last non-president to lie in state - on that you are correct.
As an interesting aside, four unknown soldiers have lain in state in the Rotunda. Vietnam in 1984, Korea and WWII in 1958, and WWI in 1921.
https://www.aoc.gov/nations-stage/lying-state
If anyone can find a pattern of qualification in these, have at it, but the only real qualification is that Congress says you can and your family agrees. Most presidents don't lie in state in the Capitol - only Ford, Reagan, Lyndon Johnson, Eisenhower, Hoover, Kennedy, Taft, Harding, McKinley, Garfield, and Lincoln have lain in state in the Capitol Rotunda. Lincoln, Garfield, McKinley and Kennedy were assassinated. Harding died in office.
Reagan's funeral was freaking production.
Concurrent resolution for Chestnut and Gibson: https://www.congress.gov/bill/105th-congress/house-concurrent-resolution/310/text
Concurrent resolution for Rosa Parks: https://www.congress.gov/congressional-record/2005/10/27/senate-section/article/S12018-2
Concurrent resolution for Daniel Inouye: https://www.congress.gov/bill/112th-congress/senate-concurrent-resolution/64/text?r=2
Concurrent resolution for Claude Pepper: https://www.congress.gov/bill/101st-congress/house-concurrent-resolution/139/text/ath?r=263
CJ's civics/history class concludes. You are dismissed. If you key my car I will hunt you down like the dog you are.