Samyang 135 backfocus calculating problems with zwo 2600+efw

Since you have the Canon version you could replace the lens flange with a M48 thread one. Firstlightoptics sell them, made by metal. Otherwise the ZWO 2" EFW adapter is the perfect fit - but only if you have the 2" EFW of course. With the smaller ones it becomes a bit more difficult, so I'd opt to replace the Canon mount and install the M48 threaded one which perhaps could give you just enough room to make it fit.

I have two myself but I have the Nikon versions and a 2" EFW, I decided to go with 2" partly because of the Samyangs I own. What I will add though, is that the "perfect backfocus" is not 44mm in your case, especially not with the larger IMX571 sensor. Filters need additional spacing (1/3 of the filter thickness apprx). Then possibly minor adjustments to dial it in, especially if used wide open which is extra demanding - even more so when using larger sensors.

So long story short, you would need either a 2" EFW and the appropriate adapter or replace the camera mount with something like this which would be the cheapest option.

Not exactly the answer you are looking for, but might be useful once you figure out the adapters: 

Even if you sum up the distances from the devices you have, sometimes there is error small errors in the distances that can screw up the backfocus.

A trick to get it right, is to use use the lens in a canon camera and focus on infinity.  After that, dont change the focus, detach the camera from the lens and set up your astronomy gear, if they are in perfect backfocus, the image should be in focus, if not you will need to play with spacer until the fixed infinity focus matches. 

In mine I had to add approximately 2 mm of spacers.

Also, linear elongated star on the corners means you the sensor is too close (solvable with spacers) and circular elongation patterns means the sensor is too far…

The only commercial alternative from ZWO would be this one: https://www.firstlightoptics.com/zwo-accessories/zwo-adjustable-eos-lens-to-t2-adapter-for-all-asi-cameras.html?_gl=1*1rir91v*_up*MQ..*_ga*NjY3NDkwMDk5LjE3MzIwMzM0MDI.*_ga_F6VGWQ0MW9*MTczMjAzMzQwMC4xLjAuMTczMjAzMzQwMC4wLjAuMA..

As mentioned above, you have to remove the EF flange and replace it by the FLO M48 type. Then you have enough space left to work your way from M48 to M42 and stay within the required 44 mm backfocus distance.

This is what I use:

1. Flange fpr Samyang 135 -> M48, 44 mm left (https://www.firstlightoptics.com/adapters/astro-essentials-samyang-lens-to-m48-adapter.html)
2. TSM48i-T2a adapter from TeleskopService -> M42, 38 mm left (https://www.teleskop-express.de/en/adaptors-10/adapter-t2-m42x0-75-122/ts-optics-adaptor-from-m48x0-75-to-t2-6-mm-optical-path-length-10401)
3. TST2V5 extension tube from TeleskopService -> M42, 33 mm left (https://www.teleskop-express.de/en/ts-adapter-39/adapter-extensions-124/ts-optics-t2-extension-m42x0-75-5-mm-length-2261)
4. 6 mm spacer, 32,4 mm left
5. EFW -> M42, 12,4 mm left
6. ASI 2600 MM Pro without tilt plate, -0,1 mm left

Example image: https://www.astrobin.com/bm8tn5

You need 44 mm without filter, a bit more with filters. 46,5 mm is way to much. It seems that you use the Canon EF flange that came with the lens. That thing has to be removed and replaced by the M48 flange in order to stay below 44 mm. The optical path of your Canon adapter is too long.