I use the word 'mitigate' not 'solve' since closed source baseband modems are a problem. Cellular traffic is off in airplane mode, but the baseband could be exploited if someone wanted to find me AND knew which IMEI to target. Because the IMEI has never been associated to me, that is a challenge.
I run my own VPN and share it with a few other people.
How confident are you in your VPN server configuration skills? Gaining access through mis-configured self-hosted boxes is the easiest attack vector usually as most people who self-host aren't experts in the software they are using leading to leaks. Besides that, do you keep a list of packages installed on your box, open ports, etc? How about security patches and regular updates as well as auditing access logs to ensure no one gained access to your box?
Squeaky wheels get the grease and squeaky nails get the hammer, but silence in a noisy forest is alarming.
How do you pay for jmp.chat? Do you trust their code to be bug-free and without possible exploits? Do they do regular security audits and code reviews? Do they have enough users and maintainers to be able to quickly detect and address security issues? Are you sure Airplane Mode turns off the baseband and cuts off all cellular communication? It doesn't, you can still emergency receive alerts in Airplane Mode. Your phone can tell exactly where you are by comparing your wifi search results + RSSIs to known public databases without even having to use GPS. How much do you trust your VPN provider to keep no logs? How do you pay for VPN?