A few things:
The minimum horizontal viewing angle specified by THX for a home theater is 26°. With a 55" TV, that would occur at a maximum distance of 8 feet 8 inches (2.64 m). I don't sit that close to my 55" OLED, I admit.
I think that you could find a 65" LG B7 (2017 model) for less than $3k CDN, but you'd have to consider your total budget.
Your SR5008 seems to be too old to handle the current 3D audio formats (Audio, DTS:X). It also can't pass through 4k video. There are work-arounds, if you can live with their limitations.
If you want to do 7.1.4 or 7.1.2 Atmos, you'd need an AVR that can do 11 channels or 9, respectively. For 11 channels, I'd suggest a Denon AVR-X4400H plus an external 2 channel amplifier. (The 4400 processes 11 channels, but has 9 onboard amps. 50W/channel stereo amps are not expensive.)
Some Atmos information: https://www.dolby.com/us/en/guide/do...tup/index.html
The "Dolby Enabled" (ceiling bounce) setups are supposed to be least desirable among the available options, but if you can't mount speakers on (or in) the walls or ceiling...
And, as pointed out in post 2, 2017 and later OLEDs don't support 3D. There are UHD TVs that support 3D, but, as far as I know, none of them are OLEDs. Example: Sony Z9D. (Also, I believe that there is no 3D source material that is also UHD. Can't get the required frame rate at that resolution.) Whether you'd be happy with an LCD TV after owning a plasma, I can't say. (Good LCDs are supposed to be very good, indeed. Their worst limitation is a limited viewing angle. Inferior LCDs also may not have good blacks.)
If you really want to go to 4k/UHD, you may have to upgrade a lot of your electronics. I hope that you have the budget for it.