However, he is not a young player and according to transfermarkt with the player is worth 13m. Leverkusen is a team that wants to hire Füllkrug for the next season because Patrik Schick got injured and still he is not in a good situation.
Bayern Munich is also another team linked with this player to have an alternative for Kane if the deal fails.
https://fcbinside.de/2023/06/19/niclas-fuellkrug-zum-fc-bayern-so-ist-der-aktuelle-stand/
https://www.getfootballnewsgermany.com/2023/bayer-leverkusen-interested-in-werder-bremens-niclas-fullkrug/
Füllkrug has been a decent striker in the Bundesliga so far. He has spent many years of his career in Germany. He has played for average or below level teams here so far. He has been really effective in scoring goals as well. However I see him as a good striker for only the kind of teams I said. I wouldn't expect to see him showing the same performance for Bayern Munich for example. He is 30 years old as well and I think he should continue playing for teams like Werder Bremen for the good of his career.
Bayern Munich shouldn't choose the easy way either. If they don't sign Kane they can surely find another solid striker who can be much more reliable than Füllkrug. However they can think of Füllkrug anyway in Choupo-Moting's place only.