.... Obviously only you can decide whether the EOS R is the best option for you. But for what it's worth, although I have no experience of Sigma/Tamron EF mount lenses I can report that the EOS R with its adapter works flawlessly with EF lenses. In fact some of the EF lenses can potentially deliver even better results due to the R body's performance.
You mention 8fps but be aware that is the absolute maximum and subject to Canon's stated as forthcoming firmware, you can't
currently get that rate when tracking. As a wildlife photographer it has been the biggest negative for me but doubtless I am spoilt by my 14fps on 1DX-2 option. Having said that, I am now finding that by rearranging my settings I can achieve 5fps and perhaps occasionally 8fps (I can't tell). Hell yeah, I used to manage very well with 7fps on my 70D not that long ago. More fps offers more image choices from a burst but that's all. I have just now tested the fps performance on a perched Sparrow by shooting a burst of 8 images as he turned his head and also his body <
Every shot looks absolutely sharp reviewing at 100% in the EVF. That was on a EF 500mm on 'tripod' (see my post #140). Also, if you read my post #125 I tested a burst of 45 frames of a dog running towards me and every one looks sharp on a handheld EF 100-400mm L II.
Furthermore, each image is about 38Mb (I only shoot RAW) and so the full-frame image quality looks very promising indeed and
@srhmoto Simon's posted images in this thread also support high image quality.
Since I last reported about swopping to a SDXC UHS-II card which can read up to 299Mb/s, I can confirm that such a card does make a big difference if you need it, just as a CFast card delivers faster performance than a CF card.
An announcement from Canon about the next R version is strongly rumoured for the end of next January with a release later in the year but we simply don't know whether it will what any of us would want.
IF it is, I'll seriously consider trading my EOS R for it.
There is no such thing as a perfect camera (not even Canon's flagship 1DX-2) but those who are actually using the EOS R are singing its praises as well as accepting its limitations. I am.