Pierre,
the SWR fluctuations you see on the NanoVNA simply show you how a handheld antenna performs in normal use! It doesn't offer fixed, good SRW to the radio. Instead it varies according to what's around, how you hold the radio, etc. The SWR you see on the NanoVNA is not the same the radio will see, because the NanoVNA is different from the radio. But there will be wild fluctuations in both cases.
To find the best antenna for your radio, don't watch the SWR. Instead experiment which of them makes the radio transmit and receive the best.
Go to an open place, and enlist the help of some fellow ham at a few km distance who can transmit at variable power, and who has a radio with a real S-meter. Test each of your antennas while communicating wth that ham. In each case, ask him to reduce the power until you hear him with significant noise, and then swap antennas and move the radio around and find out which antenna receives best, in which position. Also set your power so that his S-meter points somewhere in the middle of its range, and ask him to report the signal level for each of your antennas, while you move the radio in several ways. These tests should bring out which of the antennas provides the best performance, and whether any is more position-critical than any other.
Be sure to make these tests on at least two or three frequencies spread over the band, because small antennas are often quite frequency-selective. Testing on just one frequency might give a wrong idea.
SWR is very far from telling everything about an antenna. A 50? dummy load gives you perfect SWR, but no radiation! A very poor antenna might be so lossy that it behaves largely like a dummy load. And most handheld radios are rather uncritical regarding SWR. They need to be, since SWR varies so much depending on what's around the antenna.
When I take my handheld radio into the field, I use a telescopic whip antenna, which I can extend just the right amount for the frequency in use. I marked these lengths for quick use. It allows me using the same antenna both on the ham band and on the marine band, with good performance. And the performance is significantly better than that of any rubber duckie.
