Answers first, lots of talk later.
1. Yes, I have done this. It was efficient and effective.
2. You absolutely must think of the tones as being fluid entities that have very different realizations when they're spoken aloud. Shadowing is important because it teaches your brain to hear these different manifestations of the same "root" tone.
3. No, questioning your learning strategy and asking for advice is important. Sometimes if you keep lifting the bar without anybody telling you different, you'll pull a muscle.
Okay, more talk:
You need to have a mix of shadowing and chorusing. Maybe you're already doing that - by my definition shadowing is repeating a sentence or passage to get the flow and then moving on, whereas chorusing is an extremely intensive activity where you are slowly molding your speech pattern to mimic the native as closely as possible, for just one short phrase at a time.
Cantonese poses interesting challenges with regards to tone. Depending on where and when your wife acquired Cantonese, she may speak differently than all your learning materials. Speakers born before the 1980s (very broadly speaking) often have an "extra" tone. Young speakers in Hong Kong and Macau, as well as older speakers from certain parts of Guangdong, often merge tones 2/5 and 4/6.
All this means that if you pronounce the tones and expect them to sound like this:
you'll be at a disadvantage, because they sometimes sound like this:
and sometimes like this:
Your mental model of Cantonese tones shouldn't be rigid and precise. Depending on the way a phrase is spoken, a tone 3 can be acoustically identical
to a tone 6 in another phrase.
Fortunately, you're already doing exactly what I recommend. Chorusing is a great exercise for nailing real-life tone production. However, you need to be practicing longer. After 10-20 repeats, you may think you've got it, but if you keep pushing past that point you'll notice that you start to shift even further toward the ideal pronunciation. I've recorded myself at 10, 20, 50, 80, and 100 repeats and I can absolutely hear an improvement every time. This is true for every language. And for a single sentence, 100 repeats just takes two or three minutes.
When you're shadowing a sentence, the tones should barely even cross your mind. Sentences have flows of pitch in every language. Of course it's good to know what the tones are in their citation (single-character) form, but pay much more attention to the flow between words. That's where the extra time with 100 repeats becomes a big advantage. If you still find yourself wanting to slow down the speech, then just pick shorter or simpler sentences, or do shift+space with half the sentence.
In my opinion, you should try longer chorusing sessions, spread out throughout the day and repeating the same sentence over multiple days. Then slowly introduce your shadowing back in and reducing the number of chorusing sessions. You should notice an improvement in your fluency, and crucially, if you record yourself after a few weeks of chorusing you'll be shocked at how much better you sound.
The more time you spend on chorusing the better, especially if you've picked up bad habits over the last 10 years of speaking. After a while, though, the returns become smaller and smaller and it will serve your fluency better to move on to just shadowing. Chorusing a small number of sentences (less than, say, 50) is fine because the tone patterns are consistent between sentences. Shadowing a variety of longer phrases and sentences is crucial to get your brain used to the common sound and word combinations so you can just open your mouth and hear fluent Cantonese come out.
Best of luck!