Yeah it's more "open vs closed comb", and "small blade gap versus big blade gap" that are the defining factors on whether a razor is aggressive. Open combs and big blade gaps make for the more aggressive razor.
Both 2 piece and 3 piece razors can be heavy or light, mild or aggressive. You're better off picking something with good reviews... Muhle's R89 and the Edwin Jagger DE89 are pretty much the same razor (3 piece with an identical head) and are considered to be fairly mild although they actually shave very well.
All the Merkur models you've mentioned have the same head with a closed safety bar, the 34 is short and very heavy, the 23 is mega long and slim and mid weighted, and the 33 is kind of a mix of both, again mid weighted. They're all good, the 34 was my first razor and I still use it a lot... not hugely aggressive but not too mild, it's a perfectly good first razor.
In fact I'd say pretty much any razor is a good starter as you soon pick it up, I'd maybe just think before ordering an open comb as they can bite a bit if your technique is sloppy. They are doable though.