There's obviously going to be some preference here for brand (i.e. Shimano vs. SRAM) and how much you're looking to spend. Personally, I'd think this is a great opportunity to go up to 12 speeds and ditch the front gearing to go to a 1x system, freeing up the other side of the bars for a dropper post lever 😏 . Going SRAM Eagle NX like https://www.rei.com/product/144025/sram-nx-eagle-dub-boost-groupset is a great bang for your buck I think and includes the whole package of shifters/cogs/cranks/derailleur, although you'd have to confirm if you'd need the "boost" version which is linked, or "non-boost", which is basically the width of the frame's rear dropouts where the wheel axle bolts in. @REI-JohnJ could maybe get the REI elves working to find that out for you, I can't find a product page for the DRT 1.1 maybe because it's not in stock atm. Part of what I like about the Eagle system is you can mix and match all their tiers for the most part, so you could run a GX derailleur but an NX cassette with a fancier X01 or XX1 shifter and a GX chain, which makes it super easy to maybe start with the cheaper ones and upgrade over time. Crucially, you'd want NX for the cassette since it would work with your Shimano HG freehub body: GX and above requires SRAM's XD freehub to be able to fit the smallest 10 tooth cog, while NX stops at 11 teeth to be compatible with the old standard. The hierarchy from low to high is SX, NX, GX, X01, XX1, but SX isn't really available to buy as parts, it's mostly for manufacturers. In fact I myself just recently bought a fat bike with SX components stock (because SX is what came with the color I really wanted...) and am working on upgrading it to GX and X01 parts. The cranks can throw a bit of a wrench in the works so to speak. The Shimano Tourney that's stock you could maybe leave on if you wanted to buy the individual components instead of the whole groupset package, but the chainrings may be riveted on and nonremovable leaving you without the narrow-wide tooth profile that helps with chain retention on 1x systems, and two unused chainrings which are dead weight (though I suppose the big one could function as a bash guard...). The chainring(s) will wear out eventually so I would probably recommend including new cranks as part of the groupset to have that ability to replace worn chainrings, but regardless of if you go SRAM Eagle or another brand, your current bottom bracket is almost surely a cartridge square taper design, so you'll probably need a new BB to go with the groupset (The SRAM Eagle system uses their DUB standard, which isn't bad at $38). All that said, I've also heard good things about Box Component's drivetrains as a new competitor against the Big Two (Shimano/SRAM), which come in 9 and 11 speed variants, but I don't have any personal experience with them. Hope that helps and good riding!
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