Well your first problem is that you can't add a separate amplifier to your Onkyo. You would have to upgrade to the 705 in order to do so.
But start on the speakers first. Which Polks do you have & what if any is the problem with them? You should really try to audition in person any speakers that you want to hear. You are the one that will have to live with them so it would be better to hear them in person. If you order online & don't like them, then you have the hassle of packing & the cost of having to ship them back.
The order I would take it in is this.
Speakers first. Start with the 2 fronts. If possible listen in person to get a good idea of how they sound on multiple different brand receivers.
Next pick the receiver that sounds the best with your speakers, that also has all the bells & whistles that you want it to have. Most importantly make sure that it has preamp outputs so that you can add separate amplification!
As time goes by add your center channel & surrounds. Try to get them all the same brand so that you are timbre matched. Then get 1 5-7 channel amplifier. Buy this USED on Audiogon. I always recommend 200wpc. This is enough power to drive any speakers you have now or will buy in the future. Amp technology really hasn't changed much over the years so this is one piece of gear that is fairly future proof.
I think you are all set in the sub dept.
Finally, there is no such thing really as future proofing your system because the technology is ALWAYS changing. YOU simply have to decide what is most important for your system to have & then drawing a line & saying enough is enough.
I had my first real 2 channel system for 15 years before I decided to upgrade & join the HT crowd. However I still have the bulk of my 2 channel gear. I still have my 18 year old front speakers, I still have my 18 year old tape deck. I still have my 26 year old turntable!
I simply have spent the last 6 years slowly adding to this system which is complete & you now see in my signature.
I now have all that I want to have for an Audio/HT system. I have no more plans to upgrade unless something breaks.