Damn you did your research though. The Topping+RHead seems perfect imo unless you want the iFi features (bass boost, crossfeed, different filters to tinker with), the iFi is rather expensive unless you find a deal on it though. SDAC+O2 is the most elegant and cheap, you could see if you can find one used too if you want it more quickly.
https://www.hifishark.com/search?q=SDAC there's one on
ebay right now.
Since both the SDAC has been measured as being great and the O2 just now as well (both ~100dB SINAD iirc), I think you will be safe there. I doubt you will be able to hear differences with the Topping/RHead stack unless you want piece of mind to have state-of-the-art (Rhead has not been measured here. Reputable brand though). And again, unless you want the iFi features.