οὕτω γάρ μοι δοκεῖ ἥ τε ὑγίεια μᾶλλον παραμένειν καὶ ἰσχὺς προσγενέσθαι
is it possible to say : οὕτω γάρ μοι δοκεῖ τῆν ὑγίειαν μᾶλλον παραμένειν…
The personal construction, …μοι δοκεῖ ἥ θ’ ὑγίεια παραμένειν, is regular and idiomatic in Attic. The impersonal construction (with τήν θ’ ὑγίειαν) in this context is rare but not impossible—and sometimes texts are emended to avoid it. I think the majority of examples come from Xenophon (not here, though). It’s more common in Post-Classical Greek.
With a slightly different nuance, the inf absolute would also be possible, (ὡς) ἐμοὶ δοκεῖν (“in my opinion”), or a parenthetical ὡς ἐμοὶ δοκεῖ.
so i may come across impersonal constructions while reading commentaries on Aristotle like those of Stephanus.
Just to clarify, impersonal constructions in general are common in Attic (as you know), just not with words like δοκεῖ or φαίνεται in contexts where the personal construction is also available.
I’m not sure about Stephanus. I haven’t read much of him. You can find instances in mid- to lower-register texts in the early Imperial period, eg Shepherd of Hermas. Less so in higher register authors.