But is Russia even in decline, relative to peer countries?

Compare GDP (ppp) of G8 states now to year 2000:
USA $20,8trn 102%
UK $2,9trn 91%
Japan $5,2trn 53 %
France $2,9trn 76%
Germany $4,4trn 83%
Italy $2,4trn 48%
Canada $1,8trn 98%

Russia $4,0trn 145% https://twitter.com/ABarbashin/status/1346606126201241607

Compare share of global GDP (in nominal USD terms, G8 states) in 2018, to the year 2000.

Russia the only one which increased share, despite sanctions & weak ruble

USA 5%
UK 1.28%
Japan 7.29%
France 0.61%
Germany 0.89%
Italy 0.76%
Canada 0.13%

Russia 1.04%

One more measure, the change in global military spending rank.

2000: 1) USA 2) Japan 3) UK 4) France 5) Germany 8) Italy) 16) RUSSIA 20) Canada

2019: 1) USA (n/c)
4) RUSSIA (+10)
8) France (-4)
9) Germany (-4)
10) UK (-7)
11) Japan (-9)
14) Italy (-6)
16) Canada (+4)

One area where Russia in relative decline is population (2000 v 2019). But Germany & Japan in same boat.

USA 328.2m 16.3%
Japan 126.1m 0.4%
Germany 83.0m 1.0%
France 66.99m 10.7%
UK 66.6m 13.3%
Italy 60.3m 6.0%
Canada 37.59m 22.4%

Russia 146.7m 0.1%

One point here: The 2019 figure includes Crimea. Without it, Russia's population would have fallen by 1.3% to 144.8 million.