Not seen him, but....
Last summer Leicester signed Soumare from Lille. He was a regular starter as DM for the Ligue 1 champions. He was terrible for Leicester. So bad that Mendy, that was not registered to play in the league before January, became a regular and a massive upgrade on Soumare. Rodgers could hardly find place for Soumare in the match day squad second half of the season.
Onana is not a regular starter for Lille. A Lille side that no longer are champions but finished 10th in the league last season.
I dont doubt that Onana is talented and has great potential, but is he ready for Premier League? Can Everton spend big money to sign him and expect him to need 6-18 months before he starts matches?
Liverpool spent £40 million on Fabinho and waited 5 months before they played him in the league. Gueye was not great for Villa his first season. Veretout struggled big time.
It is different with offensive players. You can be patient with them. Give them 15-30 minutes per match. Waiting for them to be ready. You can not do that with central midfielders. You need to wait for them to be ready. And without matching in Europe, that wait can be long.