Welcome to UU!
SubCM Chandelure is most definitely a viable pick, although it might be more appropiate to consider it a wallbreaker than a sweeper. Therefore, it will thrive against bulkier builds, most notably stall, while losing some effectiveness versus more offensive teams because it might struggle to keep up with the pace of an all-out-offensive battle, needing turns to set up Substitute and Calm Mind to function properly. Additionally, the defensive value this Chandelure set can provide is highly dependent on whether you can keep hazards off the field since Chandelure is susceptible to any kind of those, most notably Stealth Rock of course. When I speak of defensive value I'm mainly referring to its ability to check Scizor and Cobalion, although of course it can also make use of its two (three should you choose Flash Fire over Flame Body, which I don't recommend on this set though) immunities. As for problems Chandelure might face: Hydreigon would be the most common answer, resisting both of its STAB attacks and threatening to OHKO with Dark Pulse. Krookodile and Mega Aerodactyl are two more issues Chandelure will have to face, both of which are capable of trapping and removing it with Pursuit (switching with a Substitute up will help prevent that!). Frankly spoken, it is not hard to revenge kill Chandelure due to its weakness to common attack types (Ground, Water, Dark) despite the respectable amount of types it resists and is immune to. Now to answer your question, reliable hazard removal is my first piece of advice, Mantine comes to mind sporting immunities to two of the types I mentioned above while also sporting good special bulk to take on the most common Hydreigon set by far, Choice Scarf (people have been experimenting with more powerful sets lately though, be mindful of that!). When facing defensive cores, Chandelure appreciates help in the form of status spreading (Toxic) and hazards of your own, pressuring them to constantly heal, which grants turns for set up. There are various options to make use of for that, an example I thought of would be Nihilego, which is able to set up Toxic Spikes while also potentially functioning as a lure for bulky Water types such as Empoleon, Suicune and Swampert/Seismitoad with Z-Thunder or Z-Grass Knot, respectively. Lastly, you should acknowledge that in a good portion of games SubCM Chandelure might be unable to achieve much if anything of what it is intended to do. Keep in mind that CM Chandelure will prefer to excert pressure on defensive cores whose passiveness it can exploit, lacking the immediate attacking prowess (albeit still respectable) and more importantly speed, which both Choice Specs and Choice Scarf try to compensate for respectively. Therefore, building your entire team around SubCM Chandelure will prove to be difficult in practise, make sure you can fall back to a strategy that is not entirely focused on breaking fat cores but instead can carry its weight versus offensive teams as well (for they are much more common than stall).