I'm level 69. My bonus round record is 6900s. Unlike the others, I usually build a lot of towers in early waves, most of them are scientists.
Mr Iron was my favorite carry tower, but he couldn't help me stay alive more than 1000s. Now i use Shadow with Knux, Abyss King and Mr Iron as supporters
I've tried the "spam scientists" technique too. The hope being that they wouldn't take kills from the carry but they would still level up. The problem I ran into is that I'm still susceptible to a "SLOW ARMOR" type creepy to march right through and zap off 40% of my health. There seems to be no consistent way to win. I suppose that's part of the appeal, but I get frustrated that it's less about strategy and more about fighting the random number generator that produces the creeps. Solara struggles against air. But bunny struggles against boss. Basically you try to play the best cards you're dealt then hope that the RNG doesn't throw out a bunch of waves the card isn't good at.
I've experimented with switching tower types back and forth to meet the wave types, but then you become seriously crippled by not being able to transmute as many items/potions.
What does your early build strategy look like? (1-40 and 40-100)
I always start with a Hitman, wait until the first mass wave, then build some scientists (5~7) and jillys (about 3). Sometime i put in a beaver or spider. I only switch Hitman to Shadow after wave 100 because Shadow doesn't strong enough to kill early horseman waves. Until I reach wave 200, I lost about 0~40% hp
To answer some of your questions:
If you're playing a chance-based carry (Shadow, Ripper, Muli the Evil Twin, Hitman, Holgar) then yes, the game can bring you close to death, especially when you're still unlocking cards and skills. The more "consistent" carries (Balu, Dark Forge, Scarecrow, Wolf, Blofeld Laser Satellite) won't get you as far in to the bonus round, but can help guarantee your success if you're struggling. While it might not be what you want to hear, I'd actually just recommend picking a couple towers you like as a carry, and then planning around that. More fun that way.
As far as planning goes, there's only so much you can do thanks to RNG elements (Which towers you draw, the upcoming waves, which items you've drawn, how lucky you've gotten with previous waves). However, in general:
-Don't spend money that can be better used elsewhere. If you're saving up for Knuperhexe, and the Dark Forge rolls around but you don't need it yet, don't buy it.
-Be mindful of having other towers steal kills. In general, this is only a temporary nuisance (Loss of XP), but with some towers, this can actually result in long term consequences (Shadow, Ripper, Holgar, Wolf). In general, when you're starting out, your best bet is to start filling in the end of the map to clean up after any creeps that survive your carry. If you're playing Darkness, be extremely cautious with Miss Jelly. While she can do some amazing support duty, she can also steal a ton of kills from your carry tower.
-Be aware that some waves simply will get by while you figure out your strategies. When I first started with Ripper, I focused on getting his Crit Chance up to the exclusion of everything, including Speed. I eventually realized this cost me basically any Mass Wave that showed up early on.
-On a related note, the items you use will have an absolutely phenomenal effect on your towers' performance. In general, Speed benefits EVERY tower, while certain towers may want certain things (Luck, Critical Chance) as well.
-Don't lose sleep over the amount of Interest you can make each round. While it might be nice to earn an extra ten gold or so, if it comes between that and possibly leaking some creeps, spend the money.
Less important, but still relevant:
-Pay attention to towers that will consume money. Muli the Evil Twin and the Blofeld Laser Satellite are both very powerful, but both can absolutely bankrupt you if you aren't prepared.
-Late game carries (Dark Forge, Balu, Muli the Evil Twin, Blofeld Laser Satellite) can take a LONG time to get in to play, so be prepared with an alternate carry in the interim. Generally a Miss Jelly, Hitman or Scarecrow can keep you going, but your best best is to try and run a second carry until you can make the jump.
-Pay attention to the position of other towers (Knuperhexe, Abyss King, Manitou, Herb Witch, Irish Pub, etc) as they can have a strong effect on your carry's efficiency.
-Just as importantly, pay attention to your carry's effect on the towers around it (Balu and Wolf). The former can disable towers that might be tremendously useful, and the latter can accidentally boost up a section that could have a negative effect.
-Late game, Damage potions become irrelevant. If you have a Grey or Blue Damage potion, swipe it up and hope for Speed or Critical.
In general though, I'd just focus on having fun. The knowledge base will come when you've had more time with the game. On top of that, as you get better, you'll also get new cards, and new skills that will make it easier to play however you want.