From the one video I watched it's pretty clear the biggest problem you guys have is with your movement and tech skill, your combos were pretty good for your level.
For the Marth player I would suggest wavedashing OOs instead of rolling all the time. Alot less predictable if used right and less lag time to punish. Also try to learn the art of Marth's ridiculous dash-dance grab and baits and know all of Marth's punishes off grabs realllllyyy well as they are one of his main ways of tacking on that %. Marth's grab range is like half the stage, you should abuse the **** out of that anytime someone mis-spaces a move against you. Try to wavedash in game a lot more for mindgames and during combos, it will help out your spacing a lot more in the long run and your mindgames. Also don't use your double jump so often for no reason, if you get hit out of it you're stuck without a jump and get messed up pretty easily. Stick more to SHFFL fair's, nair's, some down-tilt's thrown in to keep your spacing, and dash-dance grabs. Marth is at his best vs aggressive players in a wait and bait type strategy. Also I wouldn't charge the shield-breaker so much, it limits your options to pretty much just one which is like the worst thing you can do. Edge-guard using downtilt, forward smash, counter, drop-zone bair or fair depending on character +circumstance.
For Link, I would lay off the shield-grabbing so much. When you play good players who space really well and know how to avoid grabs missing that shield grab is gonna get you maddd percent and possibly a stock. Using Link's Up-B OOs is a solid punish on bad spacing and is kinda safer and is also a good kill move at high percents. Also try to recover more with the chain off stage, you could have survived some of those edge-guards if you didn't Up-B. I also think his fair and bair and under-used moves, but that's just my opinion. Both are good for spacing around shields and avoiding shield-grabs. Also crouch-cancel down-tilt and downsmash....really good against people who don't expect it. And ahh keep spamming those projectiles they're really the best weapon you got.
Also I agree with the taking up a high tier argument, at least for a while. When you come back to your low tiers it's ridiculous how much they've improved. Plus playing a high tier will get you the best tech skill, but it's your choice dude.