No advice on extending naps as I have never managed it with dd! I wasted a lot of frustration and, yes, tears on this issue. Now at 12mo she will sometimes sleep for an hour or more, more often the usual 40ish minutes.
The good news it that she can make it for longer without a crap nap these days, and she dropped the third nap at around 10m. Dropping the third nap made my life much easier.
She's also much more flexible- so sometimes she has two naps, sometimes one. Often we'll go to a morning playgroup, she'll drop off on the way home, and I'll leave her in the pram, rocking her back to sleep when we wakes after 30 mins. I normally get an hour out of her that way.
I've stopped stressing and it feels so much better. Today, for instance, she woke at 5am- I know- so I gave her a nap at 9am (45 mins) and rocked her to sleep in her pram for another one after lunch. She's been asleep for an hour-ish.
Tomorrow at her childminder's she'll probably have an hour and that's it.
I've discovered that it has very little impact on her night time sleep.
Tbh as your lo is doing so well at night (at 7mo! That's amazing!), I wouldn't bother trying to make her naps longer. She clearly can self settle, or she wouldn't be sleeping all night long, does not have props to get her to sleep etc. So you have done everything "right" (though I hate the idea of right and wrong when it comes to stuff like this), and if she's still not taking long naps that's maybe just the way she is.
Honestly? I really think some babies are catnappers. Others may be along to disagree and offer actual advice!