QRP is still very much possible, I don't have any problems with CW, FT8 or PSK31 on my KX3(10W) with a simple end-fed halfwave antenna. It's all about using the right mode for the band conditions.
For voice 100W is still nice to hav but I wouldn't call not having 50W wasting your time.
That's not how I felt when I first got into radio, though, and I wouldn't recommend that new hams go straight for QRP operation. You're just going to get frustrated.
(PS: How do you like your EFHW? My father has one and we have been really underwhelmed, noise is just unbearable compared to his vertical. For portable operation it might be okay, but I think he's going to replace it with an off-center fed dipole. I'm jealous of your KX3, I've been eying that new Xiegu X5105 and the KX2. Something portable opens up a lot of fun operating scenarios.)
-- 73 DE W9NLS
The EFHW does well enough for me but I'm out in the country so I'm probably fighting a lot less noise. 20m is usually S3 and 40m S1-S2. Love the kx3, if I didn't want to do SDR stuff with the IQ output I could have probably done fine with a kx2.
However, for HF (and at this point in the solar cycle, where anything less than 50W is wasting time), it's almost certainly going to be a cheaper and better experience to pair a used 100W transceiver with something like a RSP1A [1] instead of trying to roll your own. This is what I do; I've tapped the IF on my FT-950 and pair it with a rtlsdr for the purpose of having a waterfall. Being able to "see" the bands makes a huge difference.
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[1]: https://www.sdrplay.com/rsp1a/