There is a way to capture charts from Navionics Web and convert them to mbtiles, which can be read by OpenCPN. Since Navionics charts are vector, that capture causes the charts to lose any data that might have been in the vectors, like details about ports, hazards, etc. Discussion of it is a bit out of scope for a forum about OpenPlotter, however.
I suggest you look into a program called SASPlanet, which can export onscreen charts to mbtiles. Depending on where you are, there are also charts available from companies like O-charts and others (unfortunately, not for me here in Mexico).
For speed and reliability, it's difficult to beat an SSD drive with your RPI. There are multiple web sites out there that describe how to set up your RPI to boot from USB (in the case of 3+ and below), or boot from SD card and use an USB-connected SSD for your root in the case of RPI4. Many of us have been doing this for a long time with OP. As far as I'm concerned, I wouldn't trust my navigation to an SD card unless I had a lot of backups. I know there are high-use SD cards available that might be less prone to failure.
I suggest you look into a program called SASPlanet, which can export onscreen charts to mbtiles. Depending on where you are, there are also charts available from companies like O-charts and others (unfortunately, not for me here in Mexico).
For speed and reliability, it's difficult to beat an SSD drive with your RPI. There are multiple web sites out there that describe how to set up your RPI to boot from USB (in the case of 3+ and below), or boot from SD card and use an USB-connected SSD for your root in the case of RPI4. Many of us have been doing this for a long time with OP. As far as I'm concerned, I wouldn't trust my navigation to an SD card unless I had a lot of backups. I know there are high-use SD cards available that might be less prone to failure.