Find a CFI who is experienced in Cherokee 6s and willing to teach me in my own plane, buy a Cherokee 6 now and learn in the bigger plane.
Late to the game but this is my vote. (Besides, even if you wait, you will still need to find a CFI to teach you in your own plane.)
You have already reached two major milestones. You have your private and you are almost finished with your first transition to something different.
That first transition is a bigger deal than most realize. It's the most difficult one even when the airplanes are similar, say 152 to 172. You may have already noticed how similarly the 172 and the PA28 perform (speeds and power settings almost identical) but it also involved three changes that many new pilots find difficult - high to low wing, no "both" fuel selector setting and the use of a boost pump (especially difficult for those who learned to skip climb, cruise, and before landing checklists
), and the view of the runway at touchdown. Those are not a big deal to an experienced pilot but are typical bottlenecks in that very first change from something you know to something you don't. (FWIW, I transitioned to three different make/models in the first three months after my private; I'm somewhere in the 30s now and do quite a bit of transition training as a CFI)
But the real point is that you have accomplished that first one and they get easier with each one. Analogy: you may have learned to drive in a single make/model of car. They first time you drove a different car it probably felt really weird. Now, if you rent a car you never drove before, your biggest issue is probably how the radio works. It's not quite that easy with aircraft transitions (a subject the FAA is currently interested in) but the point is still that the transition to a Cherokee 235 will be easier than the transition to the Cherokees and Archers from the 172 because (a) you have already successfully transitioned once and the Cherokee/Archer and the 235 have a large number of similarities. Now or later probably doesn't matter much.
Continue to fly the Cherokees and Archers to build up and retain your skills until you find the 235 you want, but then learn to fly what you want to fly.