As a long time (since 1992) Mac user, I too am coming to the sad realization that Apple's pretty toys are over-priced and underpowered. I work in higher ed IT and thankfully I have access to all sorts of Macs (except Mac Pros, though that will be changing soon as I take on other responsibilities). They're great to manage and use, so long as you're not paying for them yourself...
I bought my first Mac in 1997 (PowerMac 7300 with Sony monitor, 32 megs of RAM and Zip drive) for a couple thousand-! The only other Mac I ever bought was a used G4 on eBay (and a palette of beige G3s from the university for $170 when they were doing eBay sales a decade ago; these when refurbished/resold paid for the G4).
I am writing this from a 2013 11" i7 1.7GHz MacBook Air which I recently got for FREE from a colleague who had some sort of incident involving a cat and a large drink... She was going to take it to electronic waste recycling and asked if I wanted it (oh yes please!). The washed out screen is mostly useless but when attached to a monitor, works perfectly fine (and is somehow clean inside). $12 for an ethernet/USB 3 adapter (I already have a Thunderbolt adapter for a monitor from another laptop) and I was in business with a great home setup that can even handle TG renders sufficiently.
My point with this story is, if you want a Mac, try and find a deal on a recent model used one – do not buy a new one. Tossing a SSD in an older Mac will give it a major performance boost.
Another obvious option is to build your own computer. In the office I have a 2011 i7-2600 3.4GHz 8-core Hackintosh build (I call it The Beast) in a G5 case which I bought a year ago from another colleague for $100 plus some trade items (a working G5 and some 35mm lenses). He had it running Snow Leopard which I had no use for; after some hard work and much reading of the TonyMac86 forum I got it running El Capitan reliably, and added additional hard drives for Windows 10 and elementary OS Linux and bumped the RAM to 16 gigs. It's Apple-branded hardware, so it's cool ;-).
Compared to Apple's retail prices, you can build a similarly-spec'd (but uglier) system for half the price.
In the photo you can see the Hack and Terragen 4 on the large TV monitor (also given to me!); El Cap does not currently allow decent resolution with the video card but in Windows and Linux it's perfectly fine, plus I can use the other Apple DVI monitor with those OS's as well (though not OS X). I was originally using TG under Windows but it was crashing (becoming unresponsive after editing some parameters) so I switched to OS X where I've had no problems other than having to drag the app side to side to see the whole GUI.
This Hack is the most powerful computer I own despite being it 5 years old (twice the processor speed of the Air) and is perfect for Terragen renders. The one attached is from New World Digital Art's Wild_Clouds_Pack>Tumultuous_Skies_Full_Project with customizations and took 2 hours to render today (at its original larger size). I have a slightly different vertical version of the scene ready for my upcoming ezine TG promo.
Given that Apple is making more money off the iToys, maybe they can loosen up and let macOS run on generic hardware without all the grief. After all, the more people running iTunes and the App Store, the better!