Considering that the target assumes the hit points of its new form and that the caster can choose new forms on subsequent turns, will the affected targets be back to full health of the new creature they transform into on every turn?

The language is very similar to that of the druid Wild Shape ability. When using that ability, a druid assumes the hit points of its new form.  Given that it is an 8th level spell with a CR4 shape limit, I think it's safe to assume they intended it to work this way.

What're the arguments for and against self target on this spell?

Against:  First line in the spell description  "Your magic turns others (bold added) into beasts"  not others and yourself.

For:  A poor understanding of English grammar.  Rule of Cool says, "Do it".  Table/house rules.

Master plan:

Be a necromancer

Make huge army of zombies

Use wish to cast this spell

Huge army of nigh-unkillable elephants under your control

Attack BBEG

Will this work on shapeshifters?

Yes, absolutely, the polymorph line of spells does specify having no effect on shapechangers, whereas this spell makes no mention and so will not adhere to the same restriction :)

Quote from ThefallenRoman >>

Will this work on shapeshifters?

 

 As KaptainKittens said but a reminder. This only works on willing creatures. You cannot use it on unwilling ones, so if those shapeshifters are opponents this has no effect obviously.

It states “large or smaller” so you can’t do elephants :/

unless you bribe the DM. 

This... Seems like a polymorph/wildshape mix.
but it's a 8th level spell
Dnd is confusing

Quote from Sniperwes20 >>

Master plan:

Be a necromancer

Make huge army of zombies

Use wish to cast this spell

Huge army of nigh-unkillable elephants under your control

Attack BBEG

Let me fix this:

Be a spore druid (has access to animate dead)

Make an army of skeletons (higher intelligence than zombie).

Use cast Animal Shapes at level 8.

Huge stampede of rampaging Awakened White Moose at your command.  Or whatever, you can always change them.

unfortunately elephants are Huge beasts. you could do walrus though, which is arguably much funnier

w h i t e  m o o s e  s w a r m 

A big battle is being prepared. Pretend to help. Tell everyone you'll morph them into something strong and do it. Then turn them into lemmings and hurl them off a cliff.

The spell doesn't say whether the creatures have to be willing for subsequent transformations.

Get the populace of a town to rise up against BBEG
Turn the populace into giant scorpions
Giant scorpion army

what happens to a creature swallowed by a player in giant subterranean lizard form because of this spell?

do the creatures follow commands ore behave like wild animals? e.g.  If i transform a stock of bees into a wolf pack, would they attack my enemy, me ore does the DM decide that? Do wild creatures even count a "willing" ?

There are NO CR4 Beasts Large or smaller in any book as of 2022. And only one CR3 Beast. So you'd better be happy with Giant Scorpions.

The new form can't cast spells so I think you wouldn't be concentrating the spell anymore and it'd immediately end, even though when you cast the spell you were a willing target you can see.

CR 4 large beasts includes the Giant Corral Snake from Ghosts of Saltmarsh. CR 3 Large beasts include the Giant Snapping Turtles from Tomb of Annihilation, the Bristled Moorbounder from Wildemount, the Awakened White Moose from Rime of the Frostmaiden, the Giant Lightning Eel from the Hidden Shrine of Tamoachan, and of course the Giant Scorpion.