Yeah definitely the failure to address the bullpen in 2021 is one of the things you can criticize Atkins and the FO on. They thought they could patch it up with washed up relievers and it cost them the Postseason by ONE game. Instead of taking the cheap route, they should have coughed up more for better arms, especially with that roster they had.
I look back at the roster the past three seasons and yeah I think the team in 2021 was their best chance in this window. You had a Cy Young winner in Ray, a dangerous lineup with huge power and all having breakout years, and a decent rotation with guys like Manoah, Berrios, Matz and Ryu. The bullpen was an utter disaster and its a shame they never properly addressed the BP at all in 2021.
Like you said, a lot of things can change so much year to year. Even 2022 was precious and this season as well. If this team decides to keep the status quo and sit on the fence in 2023 and say "we'll focus on 2024 more," then you have to question the FO and blame the players for the wasted opportunity in 2023.