I tried to change the gap_limit to 50000 to make electrum searching for more, but electrum got frozen for hours and unresponsive.
Yeah, Electrum isn't exactly "scalable". I remember discussion which mention Electrum uses customer encrypted JSON format.
There are 2 approach i could think.
1. Export Electrum seed/recovery words to other wallet which support seed created by Electrum. I'm aware Sparrow Wallet support that, although i have no idea whether it'll run well if you manually increase gap to 50000.
2. Export Electrum master private key to Bitcoin Core. I've seen few people claim they have wallet.dat with million address without much problem. But it requires more RAM (since AFAIK Bitcoin Core load whole wallet.dat to RAM) and you need to deal with descriptor format to perform export.