came for alternative self-training on the basics of the L-game, rated 5 stars! instead of repeating the nice words here, i also have some personal suggestions, though they're mainly design choices, but please consider if they could make the game feel better for more people.
the time before detonation is, a bit short imo. tbh i'm old (in the sense of e-sport) and noob at the L-game, and it feels no time for me to pracice considering, and i often miss combo extension by a frame (no, many). may consider a bit longer waiting, and regard the waiting (and attack-blocking) status of different clusters separately, like blocksum? and combined cluster never break up? (but the bonus mino, fun) also manual detonation by simultaneously Z+X?
the greenish skin kinda blocks my reading... and the bonus mino on it might need some design to let the player read what color it is (played 5 games, passed marathon at level11, still no clue which color's bonus mino looks like what). for example, i think icon like [< >] [<·>] (the arrowhead may poke out of the mino, with 1px black border) for this skin would be better. or use “回”like skin and stick with arrow with a bar?
the wave on the stack sometimes blocks sight too, as also mentioned by other players. i see there's option, but i think it could be preserved being also readable with some tweak.
static block requiring 3 adjacent clears to destroy... imo is a bit hard...
also on marathon, my preference on built-in singleplayer modes would be rather "pure" ones, maybe one "acceleration" with just speed-up (consider even near 20G with some entry delay) and one "marathon" with escalating static mino frequency, but no or less speed-ups. 1-clear static mino with more occurrences would be more fun imo. they could be combined in later levels though, i don't mean to ditch it completely.
by the time typing these words, i even came up with another bad idea that, being non-static, the garbage mino could change into some color on adjacent clearing.
just random thinking. but at last, i loved this game! hope there will be netplay some day. elegant design won't fail!