I use a 2018 MacBook Pro (2-core) with an eGPU (radeon rx580-based) to run TR and Zwift@4k at the same time. Zwiftinsider.com has some articles on building a cost-effective PC that might be worth a read. Most of the time in zwift the graphics processor is what determines performance. Single-core CPU performance is important when in large groups, but mostly it comes down to GPU. TR is quite light on the processor and I only get the occasional stutter when zwift maxes out one of my CPU cores when big group rides hit dirt roads. Btw, zwift@4k is awesome.
A cheap way to do it is to get a small form factor business PC for a couple hundred dollars and add a inexpensive GPU and you can get zwift looking nice at 1080p or 4k resolutions. If you go the Mac route I’d look at an eGPU such as the Gigabyte Gaming Box RX580 or Sonnet eGFX Break Away Puck as they are one piece and ‘relatively’ inexpensive. Costs can go up when you get a standalone eGPU chassis and buy a separate graphics card.
You can also search zwiftilizer for different CPU/GPU/system combinations to see what kind of performance you can expect.