I largely agree. The Commoner promotion idea just seems to complicate things for no real purpose. It was only based on the logic that that the Commoners were 'Crowned Pawns', but to really follow that logic would also require them to do an initial double push, capture and be captured en passant, etc.

So I finally settled for a Chess-like version that allows only Pawn promotion, and only like in FIDE (i.e. to Q, R, B or N). No promotion to Lion, Crowned Rook, Crowned Bishop or Commoner. And mandatory on entering the zone (as deferring seems never useful), as in Makruk. That way the rules don't have to be complicated with deferral, or with possibilities for multiple Lions.

As to a Shogi-like version: promotion of Rook and Bishop is not that strong (both gain about 2 Pawns in value). And what you say about the virtual impossibility to prevent promotion of such sliders, is true. But it is in fact also true for Chu Shogi. Despite the many steppers that tend to hang out in the zone just because of their slowness, the game will at some point reach a phase where the board population has thinned so much that it becomes impossible to control the zone. I don't know if you have played or watched many Chu games yourself, but this is part of the essential game dynamics of Chu Shogi. In HaChu I take care of it by setting the end-game value of the unpromoted sliders just a fraction of a Pawn below that of their promoted version (the actual value being a weighted average of opening and end-game value based on the squared of the population density). So that actually promoting a Rook or Bishop is hardly worth anything, and won't be done at the expense of sacrificing other material because of horizon effect. If you cannot promote 'for free' now, just don't do it, because you can always do it later.

So this concern would in fact make the play very Chu-Shogi-like (and thus less Chess-like), which was exacty the idea. (But only as a 'next step', for Chess players that master the Chess-like version, and would be interested to move on towards Chu.) What I am most concerned about is the promotion of Knights. Imposing a one-Lion rule would complicate the engine (a laziness argument...), but also lead to very un-Chu-like tactics, where you would seek opportunities to sacrifice your Lion for Queen or (Crowned) Rook when you have a Knight that could safely promote. Allowing unconditional promotion to Lion would complicate the Lion-exchange rules (but real Chu Shogi has that complication too, so that is not really an argument) but would make Knights very dangerous pieces. It is true that in Chu Shogi the same holds for the Kylin, but Kylins make up a much smaller fraction of the board population there (1/46 in stead of 2/23), and are not as agile as Knights. Possible fixes are (1) don't allow Knights to promote at all, like +R, +B, Q, K, Lion and Commoner (2) only allow their promotion on last rank (3) only allow Knight promotion if both Lions are gone.