Yes, Spalletti failed with the Italian national team after leaving Napoli, he was one step away from not making it to the World Cup (Italian fans would probably go crazy from missing out on another World Cup) and there was also a failure at the European Championship. It seems that Napoli was his peak. As for Conte, in my opinion, he shows results higher than expected and if he progresses, Napoli will have a chance for another title.
The failure to get a ticket to World Cup 2022 is responsibility of Roberto Mancini, not of Spalletti.
Spalletti is a good coach but he is not at level of Jose Mourinho, Pep Guardiola in that coach generation, and Scudetto with Napoli is his peak, I agree with you. Conte is a similar good coach like Spalletti or better a little bit but not at level of Mourinho and Guardiola or Klopp. Conte is doing good job in Napoli but I consider Conte is at same level of Diego Simeone in Atletico Madrid only.
Of course, I know about 2022. I confused it with the Euro 24 qualifiers - Italy performed extremely unconvincingly in the group stage, beating Ukraine only on additional indicators + 2 losses to England + a draw with North Macedonia (which did not allow Italy to the 2022 World Cup, which is probably why I confused it). Overall, extremely questionable results leading one of the strongest teams in the world. Yes, he is a qualified coach, but not close to the top group, that's true.