My vote would be for the FMA180 Pro. It comes with a foot and rotator, has a place for standard 2" filters, has enough back spacing for an OAG and filter wheel, an easy kit to add a focuser. You will need a Canon T-ring for you current camera but it will be easy to make the 180 work with anything you might buy in the future. The Samyang is a fiddly lens that doesn’t have enough back spacing for filter wheels and OAG, most ring kits for mounts are homemade 3D printed jobs, same for focuser mounts, you would have to buy and use clip filters. Over all much more fiddly.
Both seem to be the most popular option in their category but the FMA180 is purpose built where the 135 is adapted to Astro use. Just spend the extra few dollars for the Pro model of the 180. It’s a very nice upgrade.
As for a mount, my advice is either dive in for a proper mid range mount like an HEQ5, CEM26, GEM28 or spend as little as possible on a used one, Star Adventurer, iExos, or EQM-35. All small entry level mounts are very limiting and you will outgrow them. My Star Adventurer rarely gets used, my EQM-35 is a constant source of frustration and takes regular adjustment to continue working with any sort of accuracy, my EQ6R has been a reliable, very accurate workhorse since the day I bought it and hasn’t required a second thought.