I see 3 options:
- Adobe walled garden. You already said you don't want to go this route.
- Open source. GIMP in combination with either RawTherapee or Darktable.
- Other commercial software which doesn't insist on a subscription model.
I use all 3, and I will say right away that the Adobe platform is strictly the better option. They've been consistently ahead of everyone else for years, and their tools are nicely integrated. The quality of implementation of the healing brush, sharpening, resizing, masking, stitching, and everything else is top notch. I really like how gently they're starting to incorporate AI into their features too. What is even more important is their ecosystem of plug-ins, presets and 3rd party camera profiles.
I've been using GIMP for over 15 years now and at this point I no longer expect it to ever close the quality and usability gap vs commercial image editors. Yes, it's powerful. Yes, it has a ton of features. But I never managed to become as efficient with my time in GIMP. Everything is just a little bit wonkier, and the quality of implementation is a bit worse. Nothing major. Little things, although the absence of the adjustment layers is quite annoying. With the RAW converters I would argue that the gap vs Lightroom is even bigger, both in quality (Adobe stock profiles for supported cameras are just better, and its support for lens corrections) and it is far ahead in usability. Darktable UX in particular is extremely basic, with 99% of functionality represented as sliders stacked in a panel where even their sensitivity and range aren't optimized for usability. There's no menu and limited support for hotkeys or (surprse!) for your mouse. Sliders. Keep dragging the sliders. Great place to get a taste of the experience is to flip, crop & rotate an image to level the horizon. See how long it takes and how it's implemented. If that's OK with you, you'll have a decent chance to be happy with Darktable. I haven't tried Rawtherapee for a few years now, so I'll let other folks speak to that.
Another avenue I have explored is Capture One in combination with Affinity Photo. For RAW conversion I would actually rate Capture One as a superior solution to Lightroom, especially its support for adjustment layers, the color controls, and the collection of color presets. I love their tethering feature too, because it comes with manual focus assist which is super useful for macro work or digitizing negatives with a camera. It is available as a one-time purchase. On the negative side, I found that its color output is a bit uneven across supported cameras (Fuji is awesome, Canon is so-so). Lightroom seems to be more consistent across vendors. On the image editing side, I have been quite happy with Affinity Photo, also a one-time purchase. It beats Photoshop in terms of user interface. It is more modern and lets you do things faster, but it's closer to GIMP in terms of depth+maturity of its features, for example it still requires manual assist to the healing brush, its sharpening options aren't as sophisticated, the layers don't support some useful blending modes. It also gains and loses bugs between updates, and crashes sometimes. One more thing: Affinity Photo isn't integrated with Capture One as seamlessly as Photoshop is with Lightroom.
In my case, having all of this installed, I tend to stick to Capture One when I'm processing RAW from my digital camera, switch to the Adobe suite for digitizing the negatives where I rely on some plug-ins, and fall back to Dartable and GIMP when the pain of switching computers feels greater than the pain of enduring their UX. Like others, my best hardware runs Linux.
Sorry for the long and hectic comment, these are huge applications with nuanced pros and cons. It is really hard to generalize here but I tried. Hope it helps.