You by all means did not seem bad. You had an idea of what you were doing/should be doing with your tools and didn't try to set out a billion random steins.
This is kind of long so I hope you don't mind BIG WALL OF TEXT I wrote this as I watched, so some of the stuff I mention, you do later in the vids...so yay.
First vid -
Yeah, naked j2C is very unsafe, it's a bad habit a lot of us have starting out (well, at least for me it was...that big fat CH combo )...though I'm not sure what Rachel can really do against it lol. If it ever does get blocked, try to cancel it into 236A to make it "safer" though you really need an RC.
For blockstrings, try to start them lower in the chain with 2A or 2B. You'll have a lot more options that way than if you started with say, a C move.
From 2A you can do: 2AxN, 6B, or 2AxN, throw, or 2AxN, dash in 2A continue pressure, etc
From 2B you can do: 2B, 2B AGAIN (frame advantage stagger), 2B, dash in 2A... or throw, 2B, delay 5B or 3C (stagger), etc
Or just gatling into a JC-able move and IAD or jump away...you get the idea.
Also when you end a string with 3C, you don't have to always do 5D and jump. From max range, you can cancel into SoD and still break a primer/be somewhat safe. The safest way to end her strings, other than JC'ing is to push them out and either use SoD, or 6C, where you can then set a stein and JC more safely than 3C, or 236A, or 236D/214D if there are steins already out. In the end, anything you can do to end a string safely and/or break a primer is great.
Also try to use air throw more, I mean really abuse the heck out of this if they jump a lot and don't break it/hit you out of it, dat 2k damage (4k+ with RC combo).
Other than that your stein formations were really nice, 5D and 6D are basic and effective and cover a lot of space on the ground. Just work on your hitconfirming and better combos. I think you kept trying to do a Rachel corner loop every time you grabbed her? Try the 6A earlier (to hit them higher), that way you can pick them up after the immmediate j2C. Or just do regular 6A jC j2C 2B...corner combo.
Second vid-
2:45 - If you see them crouching, you can go into 6C for a quick setup. Also, if steins are aligned right like here, after 6C you can cancel into 214D and combo into the explosion for some extra heat/damage. During the explosion you can follow up into 5C 6C..., 6C..., or run up jump jC j2C xD airdash... etc, depending on height.
3:00 on - You mentioned this already, but remember you don't necessarily have to approach the enemy, especially with a life lead. Rachel is pretty easy to zone out, and the only way she has a chance against you is if she gets close/gets you in the corner. If the Rachel is decent, focus more on zoning them out, going in for a quick mixup if you get the opportunity, and snipping off her measly 4 primers. At long range, do stuff like 6D 5D JC 6D 236D...then you can land and do 6D 5D to keep the "laser wall" moving. Afterwards you can send out more steins and/or 236A. You can keep things going for quite a while with different stein patterns, 236D, and 236A.
Third vid -
1:20 - Oh no, don't burst in the air! Wait until you come down from Belial Edge, unless he has meter/yomi to RC he can't do anything about it.
2:40 - Again, you rushed in when you could've just zoned him out with a life lead (Ragna has to take much more of a risk to get in at such little health). If you ever have an excess of meter (you had 100 meter), don't hesitate to just throw out 1-2 steins + light show super, it'll stop them and let you set up some more stuff as your "don't come near me for free" card.
2:50 - Random gauntlet hades, isn't that annoying? Your 5C and jC can give Ragna a lot of hell midrange, and aren't bad round starters if they do random specials like that.
3:30 - After explosion, you can do 6A, immediate j2c, dash 2B 5C 6C whatever instead of an air combo, though that looked hard to confirm all of the sudden. I'm not a fan of doing full air combos since the second jC doesn't really do that much more damage and you are closer to the ground/still have an airdash after the j2C with one jump. But that's just me.
Fourth vid-
0:40 - If they think they can just sledge towards you at the start of the round, just 5C at the start until they start doing something else. 5C, 2C, walk back, backdash, and IAD back are good openers against Tager in general.
As mentioned, 236D with just 1-2 stein isn't too hot in this matchup. 214D with multiple steins is amazing though. Use patterns like 6D, 4D, 6D 4D, 5D 4D, 6D 5D, and 6D 4D JC 2D 5D along with 214D are great to mix it up and make it harder for him to just mindlessly SLEDGE towards you.
Keep an eye on his spark bolt meter, when it fills up, immediately change your gameplan. Try to send out only one stein at a time and SJC into the air. There you can send out more steins and 236D and hang out in the air while he blocks lasers. Better Tagers will keep this in mind and try to approach you while conserving the spark bolt, but don't worry about this for now
Try to confirm 214D bombs into 6B, 6A, j2C, dash 2B...combo, does good damage and hopefully gets the Tager to stop doing dumb stuff.
2D can be punished with 5C, 6C, stuff if he tries to approach with it. On IB, just 2A, 2B, 3C, 2B, 5C, 6C..., he can't do anything after it.
So yeah. Just do less unsafe stuff, practice better blockstrings and stein patterns, and hitconfirm better and you'll be on your way!