Bullethell versus title? Everything I had hoped for. Pick this one up folks. It'll take a few sets to learn it since it is ENTIRELY unique, but once you understand it the game is amazing. Get a friend to play you and see for yourself how hype it is. I can definitely see why overseas they are running friggin' tournaments of this game.
Yep, you've pretty much explained why I love Senko no Ronde so much. DUO is noticeably more beginner-friendly without sacrificing most of the depth.
To add to what you've said:
Activating BOSS mode during Vanish should be treated as a last resort. Ideally, BOSS mode should be activated when both armor is getting low and when the charge gauge is full/mostly-full, since the size of the charge gauge determines the amount of armor a "normal" BOSS has, and any armor a normal BOSS has remaining at the end of BOSS mode gets added back to the user's main armor gauge (so long as the remaining amount of BOSS armor is greater).
It's also a good idea to save BOSS stock until time is about to run out. This gives the opponent less time to recover should they manage to survive your BOSS. In these situations, activating an Antifield (bullet-cancel) just after BOSS mode ends can prevent an opponent from recovering at all before time runs out, since it pushes them away from you and prevents them from attacking for a few seconds. This is especially effective when the opponent has run out of BOSS stock.
Destroying pieces of an opponent's BOSS quickly replenishes chunks of your charge gauge, and destroying the BOSS replenishes the whole gauge. So the perfect time to counter with a BOSS of your own would be immediately after you destroy an opponent's boss.
The roster is essentially twice as big as it appears, since each character has two variants (A Cartridge and B Cartridge), and there are quite a number of differences between a character's two forms (base stats, attack behavior, BOSS behavior, etc.)
A viable strategy is "melee baiting", where one deliberately gets close to an opponent in order to force them into a melee attack, and then quickly dashing away before the melee attack connects. Thus leaving the opponent open for a counter attack. It's possible to dash-cancel out of melee during the first couple of seconds of the animation though.
It's possible to do a melee combo by initiating a melee attack with M, and then pressing S during the middle of the M melee attack once it connects. Melee combos do much more damage than a "1-hit" melee attack.
Karel can quickly reload his main weapon by dashing as soon as it's fired from a neutral stance, thus allowing you to fire two quick shots in succession instead of just one (neutral M, and then another M during dash).
Lili can charge her main shot when you hold down M, and it's possible to still activate special moves while keeping it charged at the same time (this requires you to set up a dedicated Barrage [M+S] button on the controller). So a good combo for her is: start charging M -> full-circle M+S special -> release M.
Ernula can still attack with M while her S lasers are active. So you can sandwich an opponent between her S lasers while firing at them with M.