Call it hunch, but I don't think the Kanto Dex was made wih Steel types in mind.
Scyther is a must have because of Scizor. Let's see what it learns in Let's Go... Steel Wing isn't a TM in Let's Go?! There only TMs are Iron Tail and Flash Cannon. Um... Okay. No Scyther. I guess that makes this more challenging. So, the Steel Moves in Let's Go are... Iron Tail and Flash Cannon. They even cut Iron Defense?!

Onix
-Iron Tail
-Toxic
-Stealth Rock
-Dragon Tail
This is the most obvious non-Steel choice. Onix would evolve into Steelix in a game that isn't restricted to the original 151 and it learns Iron Tail through Level up.
The moveset isn't just an annoying tanky moveset for the sake of giving a Steel team's defensive Pokemon an annoying tanky moveset. The non-Steel moves are desperately needed for Onix to do anything. Its stats are bad enough that I seriously considered Flash Cannon instead of the much less accurate Iron Tail because Onix's base 30 Special Attack isn't much worse than its Base 45 Attack! (I went with Iron Tail because Onix is the only Pokemon in Let's Go to learn Iron Tail without a TM.) Toxic is more effective than any attack, Stealth Rock has an effect that lasts the entire battle, and Dragon Tail forcing switches makes Stealth Rock way more damaging than Rock Slide and lets Onix use Toxic on more Pokemon. It's a mean moveset limited by Onix only having two Base Stats above 45. Just have a Pokemon that can outrun Onix's Base 70 Speed and KO with any Special Attacks and the moveset will mean nothing!
I'd usually lead with a Steel type for a Steel team but Onix evolves into a Steel type, I was giving it Stealth Rock either way, and I want what's clearly the worst Pokemon to get KOed first. (Sorry Onix fans for the hate, but I'll gladdly sing its praises when you give it a Metal Coat.)

Alolan Dugtrio
-Flash Cannon
-Dig
-Substitute
-Sucker Punch
Dude... I'm trying to make a defensive team. Why'd the frailest Gen 1 Pokemon get a Steel Regional Variant of all Types?
Dig's invincibility turn and hoping to use Substitute on a turn the opponent isn't attacking are about as good as you can get with Dugtrio defensively without spamming Protect. Sucker Punch's priority can also help with not taking much damage if it KOs before damage is dealt to them. The idea behind this moveset would be better with Toxic, but Dugtrio surviving a turn to be able to use it isn't very likely.

Porygon
-Iron Tail
-Psychic
-Toxic
-Recover
With Steel being the second most associated type with the modern world and technology (Electric being first) Porygon being man made made it a better thematic fit IMO than pretty much any other Pokemon that could learn Iron Tail. (In other words, any other Pokemon with a tail.) You'd think I'd keep complaining about stats with Porygon, but its stats aren't bad for a Pokemon that can evolve twice and I like Porygon enough to not want to bash it too much. That said, I considered Foul Play so Porygon would be able to not use its stats. :P
Psychic is used instead of Tri Attack because 5 Pokemon on this team are weak to Fighting. (It's not a very good team, but you could've told that by the use of Onix. :P) I don't think I need to explain the last two moves when trying to be defensive. Stall is fun! :D

Blastoise
-Flash Cannon
-Scald
-Dragon Pulse
-Rest
Blastoise learns Flash Cannon through the Move Reminder, making it and Onix the only Non-Steel Pokemon able to learn Steel moves in Let's Go without a TM. For the choice of STAB, I had to go with the one that can make it easier to take hits for a defensive themed team. (if I can even call it that after the first three Pokemon...) I'd usually go with an Ice attack for any Water Type, but Steel and Water are already both resisted by Water. I went with Dragon Pulse instead because it's not resisted by Fighting and I din't want Earthquake when two other Pokemon on the know Ground attacks too. Rest is for the sake of more than just Porygon having a recovory move and Blastoise is probably the best Pokemon on this team for tanking hits.
Blastoise isn't Mega because it isn't Steel.

Alolan Sandslash
-Iron Tail
-Ice Punch
-Earthquake
-Bide
Two STAB attacks and an attack that's STAB for the original Sandslash. Not much to say about that. Counter replaced Metal Burst as a Level Up move in Let's Go and with Sandslash's Defense and resistances, I couldn't say no to a move that not only only works if you endure a hit but retaliates with even more damage than you take.

Magneton
-Flash Cannon
-Thunderbolt
-Tri Attack
-Thunder Wave
For the Ace, the only Steel Type that isn't a Regional Variant. Magneton's coverage options are limited to Normal attacks and Toxic. Magneton's stats are more offensive than defensive and Normal is good enough because it isn't resisted by Electric, so Tri Attack works. There's so few fourth good move options, so I went with Thunder Wave for even more status problems after Toxic and Scald. I would've gone with Metal Sound instead for pseudo-setup, but that's not in Let's Go.
This team could easily get 6-0ed by a Fighting type and honestly feels uninspired to me. Maybe being over analytical with my approach to team building can cause more problems than it solves.