Should I switch to mirrorless?

Two years ago, I splurged on a Nikon D5200… And then a whole new world opened up to me. I swapped up for a Nikon D7100, and I've been loving it (I know the D5200 and the D7100 have the same sensor… But dang are they different to use).

I own two lenses: a 35mm prime and a 11-16mm superwide. I've achieved great results with both.

But I noticed that I do a lot of indoor or dark photography. It's rare that I'm outside in full sunlight. Compared to full-frame shots, I'm noticing my photos suffering from needing to be at high ISO and low shutter speeds on my crop-sensor.

I'd love a full-frame camera, but I don't have $4000 to spend. On the other hand, $2000 for a full-frame mirrorless with what looks like a decent kit lens sounds very attractive, especially with the phenomenal results I've been seeing with the new A7 Mark II.

And I've also started dabbling in video, and I'm told Nikon (insofar as my D7100 is concerned) is inferior.

My reservation, obviously, is switching to a whole new platform. In addition, my eyesight has somewhat deteriorated and I find viewfinders difficult to use. I rarely produce sharp pictures using viewfinder and manual focus. If I have to use manual focus, I always use live view.

Any thoughts on the matter would be great!

Why switch? Just get lenses that have image stabilization. Or get the A7II and a Nikon-to-Sony adapter and use all of your existing lenses on the Sony. I don't see why you'd want to switch when the Nikon will outperform the Sony in some situations. They're not the same kind of camera so unless it's a budget issue, having and using both is a logical option.
Still another option would be to switch to Pentax which has image stabilization built into the bodies. This means that you can have IS with all lenses that mount onto the Pentax, even lenses made in the 1960s. You can use any and all primes and zooms and still have IS. The Sigma 18-35 f/1.8 lens, for example, doesn't have IS, so unless you've got the A7II or a Pentax DSLR, you're going to have issues with low-light photography when using Nikon or Canon.
If you don't need the flexibility of the a DSLR, then the Sony A7II is your best option. If you need the features of a DSLR, then go with the Pentax.

If you go mirrorless with Sony you will gain the huge benefit of being able to use Focus Peaking for manual focusing. It makes it as easy as it was with the old split-image viewfinders on film SLRs.

You don't have to opt for the dinky little A7 series, there's still the Sony A99 as in the link.

But try a tripod option first just in case it helps enough, but if you don't want subject movement in dim light and don't like high ISO noise then investing in a 35mm sensor camera may help. Why not borrow one and see first?

I caught an article on You Tube from a travel/landscape photographer that made the switch, he made a decent case for switching, you can get your complete outfit in one case, the lenses aren't as big and he personally liked the benefit of the kit being lighter, easier to transport, more portable.

He did admit that there's a slight trade off in quality, and that for him the gains made the switch worthwhile, no clients have noticed the change.

Invest in a good tripod and external flash. Pouring money into an FX camera and lens to get maybe an extra stop or even two of ISO won't eliminate the need for a tripod.