stay away from Marchessault. the guys is good, but he one of the older UFA out there and god know if he'll be useful when the team is ready to compete.
The most logical option for this Summer is to trade for another potential top6 young player, draft a Demidov or Lindstrom, play Roy in the NHL. Sign Slaf to a Suzuki like contract. Weaponize the cap space by taking on a 1yr bad contract.
Next summer, you'll have a better sense of if Dach can center a line, can the new young player fit in the top 6, will Demidov/Lindstrom develop as hoped and be a top 6, can Roy play top 6 or or the 3rd line. Once you have that info, you go into the UFA season knowing if you need a C, or a W, or a D, need more size, skill, etc.