Its still winter season so TUI should be able to cover their own Ops - but if the MAX no fly ban goes on for ages then yes their S19 season will be a trial for TOM/BY and their TUI Benelux counterparts to cover...
Maybe they are de-mothballing any 757's they have just sent back as we speak.(and keeping the 767's)
As for DY/D8/DI/NAX they have a much bigger MAX fleet based in the UK and they do fly charters for TUI Hols.
Enter Air will not be able to fly S19 for Sunvil Holidays own Greece and Greek Islands charter series with any of their MAX fleet if the ban goes on into the Season.