You're assuming they have the capability to design their own refit, when even the Vulcans needed Starfleet help to do so.
I think giving them a stock Constellation is really feeding them a turd and calling it lunch. This is not a vessel they're going to refit at some distant date in the future. It's something that they're going to be building starting right now, and a lot of it. By refusing to hand over the refit designs, you're condemning them to build garbage.
Either be honest, or stop trying to make up your own estimates for what other people are and are not assuming. Doing the latter, while not doing the former, is extremely poor form.
It wouldn't be that hard for us to assist the Tauni in designing a different refit. You know that. We've done more, and stranger, to win over affiliates.
The 'turd' ships you affect to despise (
lovely choice of well-poisoning imagery, by the way) are better than what they now have. They present considerable potential to be upgraded, either along the lines we've already charted or (with further help from us) along other lines.
Unless the Tauni are suffering from an overabundance of production capacity for T0 and T1 ship parts (in which case why are they even talking to us?), they need to work up to constructing modern ships in any case. That involves working on TOS/TMP-era designs as a transitional phase.
The
Patroller-A and
Constitution-B designs present this possibility in an interesting way because they are, at heart, TOS-era designs (T-2) that get retrofitted and amended with T0/T1 parts to increase performance. The
Constellations present a different route, representing a complete T-1 ship that provides a sound basis for modern or near-modern shipbuilding techniques as a whole, while having somewhat more similarity to the modern designs the Tauni eventually want to construct.