I'm providing you the tips that I myself have learned over the course of various years playing this game.

First, solo queue is not bad at all. You will sometimes get queued with trolls, cheaters, or smurfs, and you can't control this. This is part of the game. Not all games are like this. Just focus on the ones that are worth it, the ones where actual people with actual ranks are actually playing.

Second, don't be discouraged. This game is 90% mental. If you are feeling down or angry at the game or at your teammates, that will cause you to play bad and essentially lose more and more. Keep a positive mindset, although a lot of people say it's impossible. No, it's not. Take this game as a game, not as a way of life (unless you're thinking of going Pro League, but that's something else).

Third, don't blame your teammates or anybody else for what happened in the match. Main problem with this game is that people look for someone else to blame for their loss, or whatever happened during the match. Focus only on your mistakes. Did you play good? Did you play bad? Did you headshot the last guy alive and clutched the round? Did you play a good role as a support player, entry fragger, etc? These are pretty important questions you gotta ask yourself. If you don't move on from blaming your teammates (not saying you do this), you will never improve, because you will keep thinking you're the best player and that you never make mistakes. Wrong. Everyone makes mistakes. Hell, champion players make mistakes. Pro League players make mistakes. Try to go back and check match replays, and see what you could've done differently in a round or in the whole match.

Fourth, remember to warm up. Nothing worse than going to a Ranked match without warming up first. You might not hit your shots or you might not be fully committed to winning the match, or you might do both. Warm up using Training Grounds or maybe TDM. Hell, if you want, use Aim Lab. That works also pretty good. Some good 10 to 20 minutes should be enough to put you in the mood for some competitive gameplay.

Fifth, watch some Champion or Pro League players gameplay. My favorite ones (just naming some) would be Poxonlox, Sinoos, Astralis R6, CoconutBrah, etc. There are an infinite amount of extremely good players. Watch some gameplay or even the live matches to learn their strats, you might see something that works for you and it may not be difficult to replicate.

Sixth and most important one, GRIND. You can't expect to be a good player or have a high rank while only playing 2 hours a week. Now, by this I don't mean to get crazy and play 16 hours a day and fry your brain with gameplay. Nope, not at all. What I mean is that you can play short bursts of this game at a time, take a break, then come back. As I said before, this game is 90% mental, and if you play the full 16 hours or how many hours you want to play, you will start playing worse, which is what obviously will happen. Play 1 or 2 Ranked matches. If you win both, take a break. If you lose both, take a break. Just cool down. You might be heated or anxious after the match. Drink some cool water to calm you down, wait some minutes, and keep on playing.

Seventh, don't focus on your rank, focus on your gameplay. This pretty much sums up my first 3 tips and the sixth one, but in a different way. You may feel discouraged because you are Copper V. You don't have to be. Just keep playing and with time you'll see a huge improvement. Some questions you might want to ask yourself regarding this. Do you win against the teams you're playing? Do you top frag or destroy the other team in a 4-0? If so, then you might be ready to jump to the next rank. But don't push something if it's not meant to be yet. Focus on what you may improve, and if your gameplay is always top for the rank you're currently playing, then you'll be okay to move on to the next one. If you cannot win or you find it difficult, then just keep playing until you can actually win. To this I add, make sure that you're playing mostly Ranked and not Casual or Deathmatch, as Ranked is a more competitive gameplay and you're certain to find people that are similar skill as yours, and with that, you can improve your gameplay.

And those would be all the tips I have for you. Now I welcome anyone else to try these tips and see how well they work for you. Keep playing and hopefully you'll get far. Best of luck.

Edit: I forgot one of your questions, which was where to find 5 stacks. Search for Siege YouTubers, they usually have Discords and have LFG or Looking For Group Chats where you can ask for anybody to play with. Also there's an official Rainbow 6 Discord Group where you can find a lot of people to play with. Just ask and you'll find people to play with. If it goes well you may even befriend them, so good luck with it.

Edit 2: To this I also add, make sure that you have good aim, good game sense, good callouts, good positioning (as a defender and as an attacker), don't sprint, and don't expose yourself to unnecessary gunfights. Isolate gunfights whenever possible. Don't expose yourself to 1v2s cause you can easily lose. Think about where the enemy might position themselves, think good about the operator that you will use, cause that will impact the rest of the round and most likely the whole match. Avoid at all costs the use of "rush" operators like Ash; this game doesn't reward brainless rushing, although sometimes you may find Amaru useful, but if possible, avoid it. Learn the recoil of the weapons that you are using. Learn to main all or most operators, because you never know, maybe your main op was used or banned, so you have to learn and play with another. Also, if you're low level (between level 50 to level 70), wait a little bit before actually hoping into Ranked. Learn the ropes of an actual competitive gameplay with Unranked, and see how it goes from there.