You can use your own entropy on any BIP39 supporting wallet by downloading https://iancoleman.io/bip39/ and running it offline. This also has the added advantage of being open source, so you can be sure the seed which is displayed matches the entropy you have generated. You haven't yet answered my question above about your claim that you are open source, but you provide no link to any code repositories. You can even just calculate it manually if you wanted - the only program you would need is something simple to perform a SHA256 hash for you to calculate the checksum.

At the risk of security. Various studies have shown that Google can track phones even while they are in airplane mode, and as soon as airplane mode is turned off, data which has been recorded while airplane mode was on can be transmitted.