https://github.com/FarCanary/ElectrumSeedTester
You download the stand-alone html and run it offline.
It might be useful if OP know about JavaScript, however my past observation that the software create single HTML with 109K line, which makes it less useful.
I made quick check using GitHub compare feature at https://github.com/iancoleman/bip39/compare/master...FarCanary:master. At a glance, i didn't see anything which could steal your seed phrase. However, MrFlay made new file (ElectrumSeedTester-standalone.html) with 109,370 line.
I doubt anyone would bother read 109K line, so it'd be easier to inspect network activity when using this tool.