With the money that Bavaria received from the sale of Lewandowski, it is impossible to buy anyone at all who would correspond to the level of Bavaria. But Bavaria has no shortage of money and can afford much more expensive purchases and I hope they accept this reality that the "cheap" options have run out. I think that Füllkrug is just a cheap option that doesn't fix anything. If you make a purchase, then it must have some meaning.
Bayern Munich is very careful in spending their money, they will not spend money on players of standard quality.
Bayern Munich is not a club that is in financial trouble, the sale of Lewandowski is not much with the cash they currently have. If they bring in players for short-term targets as a coating option for injured key players, they will choose free transfer players. Reportedly Bayern Munich are starting to be interested in signing Ronaldo after his status as a free transfer player, Bayern Munich does not need to spend money to sign Ronaldo as an option to replace the injured Mane.
I don't know where you heard this because just a few days ago Olive Kahn, on behalf of Bayern, ruled out signing Ronaldo.
Free players are no longer free - this is the new reality. Look at the size of Mbappe's or Haaland's signing bonuses - previously such amounts would have been record amounts for a regular transfer, but now it's just a signing bonus.