[Rebound]

I dunno, maybe I am just lazy. But in order to play options 3, 4 or 5, additional structures have to be added, increasing complexity. I admit I haven't really tried any of the other methods to determine their effectiveness, so I don't claim my opinion is based upon experience. But many partners I have encountered on bbo have enough problems dealing with the conventions they already play without adding more to the mix. It might be different with a regular partner.

[BridgeBuff]

I prefer option (6) which you missed :-). Open 1C with either 3=3 or 4=4 with balanced hands where I intend to rebid 1NT (or raise pard's M with four, of course). If unbalanced I will open 1D when 4=4. As a weak notrumper, you want to give pard the option of bidding 1D over 1C instead of some nebulous 1NT bid, and have the strong hand exposed. If the opps interfere, I can often introduce some kind of double to imply diamonds.

I want to avoid 1D - 1NT || 2NT (opener rebid to show 15-17) which gets you too high, and wrong declarer when 1C - 1D || 1NT settles you in the right spot (although more open to overcalls).

There are different implications for 15-17 notrump openers, which I try to avoid being.

[helene_t]

Playing 2/1, you can use the same methods over a 1♦-opening as over a major suit opening if 1♦ promises a 5-card. I would still open 1♦ with 1(♠)444, though.