You make a reasonable observation. The standard form we think of is melodic antiphon with psalmtone verse(s), such as found with the Introit and Communion. The Gradual, Alleluia, and Offertory take the opposite approach (though in the case of the Offertory, the music for the verses is only marginally more difficult than the already difficult Offertory itself). I'm not sure it's absolutely correct to say that the Gradual verse was sung by a soloist, since the entire chant was probably considered soloistic, compared to simpler genres. In the traditional practices I've seen, the entire Gradual is sung by a select group, which gives it more of the meditative, reflective quality for which it is intended.

I would personally find it strange to simplify the Gradual respond with a generic psalmtone, and leave the verse melismatic. But if you really have only the one singer who can handle the authentic chant, you're well within your rights to experiment. In that case, you should at least get your full schola to sing the end of the verse, as marked (*) in the old books (not the new, sadly). If you follow the ancient practice of repeating the respond after the verse (as the new books seem to advocate), I'm afraid the return of the psalmtone will sound very incongruous. A better option would be to find simpler, but through-composed, music for the respond that your schola can sing (chants abreges might work), to which you can add the authentic verse, followed by a repeat of the respond.