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 as of 2023, Canon has officially discontinued the EOS M system; and native mount EF-M lenses cannot be adapted to EOS R.
Most folks on messageboards [in 2023], who contemplate moving from a Canon dSLR to a mirrorless system tend to look at the EOS R because they can adapt their current glass. Or if they don't have lenses they want to take with them, contemplate moving to Sony E (A7 and a6x00 bodies), Fuji X system, Nikon Z, Olympus/OM or Panasonic micro four-thirds, or Panasonic S [full frame; Leica L-mount].
Which system they choose depends on personal preferences in handling, sensor performance and size, system size/weight, lens selections, and features. And different shooters are willing to make different
tradeoffs. For example, micro four-thirds uses a smaller 2x crop sensor which impacts dynamic range, resolution, and noise performance, but can create a more compact system. And being the oldest mirrorless system has the widest/deepest used market, but also possibly the least secure future). Panasonic S isn't that popular among stills shooters, but is highly attractive to videographers. Fuji X is incredibly stylish and popular, but doesn't have full-frame options. Canon R and Nikon Z don't have the breadth of 3rd-party lens selection Sony E does due to being newer systems.
Unless you know what tradeoffs your giftee is willing to make? You could choose something that's just plain wrong.