It just depends on himself and also the other players and the coach's strategy. There's no guarantee he can do well in the easier leagues if other players can't contribute much to the team because of his quality. So I think it will be a factor that will determine Suarez's performance later.
I think he will have a good performance at Nacional, because he will play with a strong team in the Uruguayan league. Usually, players who return to their old clubs will be key players there. We can look at Carlos Tevez at Boca Junior as an example.
I don't understand why some Laliga, Serie A or Premier League teams don't offer him a contract. He looks still capable of scoring goals as an experienced striker and he's not done yet. But anyway his age might be the reason a top league team as mentions at the top and therefore they didn't offer him a contract, it makes sense in my opinion.
Age is not a major barrier for world class players like Suarez, have you forgotten that we also have Ronaldo and Ibrahimovic who even now play for big clubs?
Maybe Suarez gets offers from big clubs, but he prefers to play with Nacional. That makes a lot of sense.