any Android app that permits charging when %battery hits 25-30% (configurable) & stops charging when it's 80%?

Ok, so if I'm on other phone brands, when the built-in
battery can't hold charge very well anymore (my spouse's
iPhone 6+ barely lasts 5 hours while on standby after the
phone was used for 2.5 years), the next options will be:
a) go to the workshop to have the built-in battery
    swapped
b) get one of those casings (eg: PowerBear): so these
    casings should not be obtained for new batteries as
    they'll charge the battery even if it drops by 1% &
    this dramatically increases the discharge+charge
    cycles of the built-in battery?
    I saw one ad for such battery case that claims it
    increases lifespan of built-in battery but I beg to
    differ.

Much appreciated any comment on item b above,
agree or disagree

One other question: this feature with ZenFone is available
even with the older Zenfone 2, 3, 4 or it has to be 5 & 6
(with newer Android versions) ?

I saw one ad for such battery case that claims it
    increases lifespan of built-in battery but I beg to
    differ.

It will not decrease or increase the lifespan of the internal battery. It just adds an extra power source when you cannot find one. Of course, you need to find and use one battery case which is MFi certified.