Reddit mentions of Hon&Guan Straight Duct Reducer and Increaser Adaptor Connector Inline Heating Cooling Ventilation Duct Fan 6" to 4" (150mm to 100mm)

I have the smaller k40 and the first upgrade I did was printing an exhaust adapter and getting a new extraction fan since I didn't feel like potentially burning my house down with the included fan. I had to fix the wiring out of the box since the fan wiring is crap and would have definitely caught fire at some point after firing it up.
I went with a 440 cfm 6" fan from amazon. The adapter I printed for the back of the k40 was a 4" so I also got 24-6" adapters from amazon to connect it to the fan and then from the fan to the window. I used it on both sides since I had already bought a good length of 4" hose and didn't want to also buy 6" as well. I put weather stripping on all connections in the exhaust line so I have virtually no smell in my office when running it.

I got an air pump on Amazon and3D printed an adapter I got from Thingiverse for it to direct the airflow in line with the laser. I bought my tubing for the air assist at Home Depot.
I also added a drag chain to keep the hose out of the way. I bought mine on Amazon but you can print that and adapters yourself if you're up for it.

The next upgrade I did was to add the mA meter since I have a digital panel on mine. That gives a much more accurate read on the power when using the laser.
I then upgraded my control board to the Cohesion 3D Laserboard ($200)and started using Lightburn ($40) for the software. That software lets you do a LOT more with the machine and you can also add a camera to get a view of the print area in the software to help with laying out materials.

Because the new board has power management built-in I set the k40 to the max power I would want to use on a project and then control the power levels from the software. This also lets you do true grayscale images and 3D engravings.
Because I don't like to have the exhaust and air pump running all the time I also got some wifi smart plugs. Each plug has two outlets and each of those can be controlled individually. They can also be controlled by an echo so I have that setup as well. Since you can also group smart plugs in the Alexa app I have one set up for everything and when I'm done working on a project I can either tell my echo to shut off power to the group or hit the button in the app to shut it all off at once.

For water cooling I'm using a home depot bucket however I have hose couplers installed in the lid for the water lines so if I want to move things around I don't have to completely disconnect everything and pull the pump out of the water. I just pull the hoses off the top of the lid and the pump stays in the bucket. I have the plug for the pump also going through the lid with a rubber seal around it. Depending on how hot it gets in the workspace a water chiller might be recommended.

The latest item I bought is a lab jack which I'm going to use to make a manual adjustable bed. If you can make one yourself or buy one at an affordable price an automatic Z bed would be great if you want to work on larger materials. A manual adjusting bed is fine for me now though.
I still need to purchase a new bed for it and get rid of the crap that came with it. I might try to salvage it though by drilling holes in it and installing metal spikes to rais the material up off the surface.

On my list of to-dos is to get a better lens and upgrade the mirrors. I haven't had an issue with what I have now so it hasn't been a priority.

I hope this helps with your project.