Old Onkyo Upgrade -- Denon vs Cheaper? (I want simple Atmos)

Currently I have the Onkyo TX-NR708 7.2 Channel Receiver (HDMI 1.4a) which has actually had zero issues for me, despite some people being unlucky with their units.
https://www.amazon.com/Onkyo-TX-NR708-7-2-Channel-Discontinued-Manufacturer/dp/B003N197CU

110 Watts per Channel at 8 ohms, 20 Hz–20 kHz, 0.08%, 2 Channels Driven, FTC

I have my fronts bi-amped (polk monitor 70s).

I also have a 75" 4K HDR TV, and I've been using ARC to get around my old receiver with is not ideal.

I have long term aspirations to do a major upgrade someday, maybe 5+ years off, to premium fronts and a 7.2.4 or greater atmos setup. In the interim, I feel jealous of Atmos and I want in on the party. I am moving to a new space, so I am considering an upgrade to add some ATMOS Enabled Upward firing speakers (It's a rental - I can't go drilling holes in their ceiling!).

For reference, this thread has pictures of my proposed room layout (I am moving to a new place, which has inspired this):
https://www.avsforum.com/forum/89-sp...orth-room.html

As you can see, the home layout (and my disposable income) prevents me from investing into a true, proper theater setup. But I'd like something that will serve me well for the coming years until I can someday splurge on a killer dedicated theater room in a large house, life circumstance permitting.

Here are my questions:

1. Given the mid-low range quality of my Polk speakers (Monitor 70s), is it worth it reap the benefits of the better amplifier in the more expensive $1400 Denon X4500H? If so, I understand its a great piece of kit! I just don't want to overspend here now if I want hear much benefit with my current speakers.

https://www.amazon.com/gp/product/B...5GS5T&linkId=1269bb2c00de4203f23b26d27632d002

2. One concern on the X4500H is, HDMI 2.1 is so close with higher refresh support and all that jazz. I am not exactly in a hurry here. So if next spring, we are going to have a refreshed 4500H model with a better HDMI spec I can absolutely wait to spend big and make sure the receiver will be with me for future video gaming at 4K 120fps and so forth.

3. Given the current state of my speakers and the great value you can get these days, should I just snag a $350 Onkyo or Yamaha 4K HDR receiver with Atmos and then sell it in a few years for a Denon when I do a more sweeping upgrade (say to $2k front speakers, etc.)?

4. I don't want to take any steps backwards from my current setup, so I'd want a receiver that matches the power output and ability to bi-amp like my current Onkyo has. Any recommendations?

In summary:

I am thinking of going to a simple Atmos setup with upfiring front speakers. Someday, I'd love to get a bigger house and do it right (7.2.4 or greater setup with premium speakers and amp, etc.). Today is not that day. What should I get for my receiver? Onkyo/Yamaha for $350 range, spend big on Denon X4500H? Or wait until next year to get a HDMI 2.1 Denon model and add Atmos upfiring then?


PS - another companion thread discusses the rear surround & atmos enabled speaker upgrades that I am considering mixing and matching into my system:
https://www.avsforum.com/forum/89-s...iring-replace-rear-surround.html#post58501602