dSLRs cannot use add-on electronic viewfinders. They don't have the communication contacts in the flash hotshoe to "talk" to one. The Canon EVF model you're looking at is only for Canon's mirrorless cameras that do not have a built-in EVF. So, that would not be a good present.
Typically, attempting to purchase a camera as someone who doesn't know anything about cameras for an experienced photographer who already owns camera gear can be something of an expensive minefield. This is like clothes shopping. You have to find something that fits the person's personal style, preferences, budget, and physique, and the chances of screwing it up are legion because of what you don't know.
It all comes down to personal preferences, usage, and budget. And a camera is not the only really expensive thing a photographer might want.
The general advice for folks on boards is that you a) either do a gift certificate with a budget limit to the person for them to spend what they want on camera gear, or b) to give up the happy picture of a cool surprise present, and involve them in the selection process. It's not as fun as surprising someone with a big extravagant gift, we know. But it's also better than disappointing them with a big extravagant gift they actively don't want. She may not want a camera. She might want a subscription for Lightroom/Photoshop, or flash gear, or a better tripod or tracking head or...
If your wife is shooting a Canon dSLR, she might prefer having additional Canon dSLR gear rather than moving to a different system where she would have to leave behind all her Canon dSLR lenses and lights, or would have to adapt all her lenses. She might prefer getting a specialized Canon body that's been modified specifically for astrophotography. She might prefer going Sony or Fuji mirrorless rather than Canon. You won't know until you ask her.
You also linked to three completely different types of cameras. The Powershot is a fixed-lens P&S camera. It's a high-end enthusiast compact with a 1"-format sensor, but it's still got a smaller sensor and no option for changing lenses like a Canon dSLR does. It's more compact and convenient, but it's not as flexible or powerful as a dSLR.
The K-70 is a Pentax dSLR. It uses a completely different lens mount from her Canon EOS camera, but is pretty similar in how it works. Whether she'd prefer it could be questionable, particularly since she's already used to her Canon.
The M6 is a Canon crop-body mirrorless camera, and would also be a complete system shift, but she could use her EOS lenses with an adapter and retain full lens function (e.g., autofocus). It's much smaller and more compact, and has similar function to Canon's dSLRs. But the future of the system is currently in doubt, because of the success of the newer EOS R full-frame mirrorless system, and the incompatibility of the EOS M and EOS R mounts making some folks wonder if Canon's about to create a crop EOS R-mount system and discontinue the EOS M line.
Most folks on messageboards posting about it around the time of this writing (2021), who contemplate moving from a Canon dSLR to a mirrorless system tend to look at the EOS R, Sony e-mount (A7 and a6x00 bodies), or the Fuji X system (no full frame options, but really nice vintage-style bodies and haptics/handling). Which system they choose depends on personal preferences in handling, sensor performance, looks, and what features are important to them as a photographer. If someone's willing to sacrifice sensor performance for a smaller more compact system, there's also micro four-thirds mirrorless. If they're more interested in video, then Panasonic or Sony might look better than Canon.