Even if it's 10s or 100s of thousands it can be done with electrum, you just have to import a couple of thousand addresses and check. If none of them come up as loaded then you delete the wallet and do it again.
There's an exception to be aware of: Electrum doesn't show the "send to pub key" balance. You could be overlooking an old 50 Bitcoin block reward.
True, but for some reason I don't think that is an issue with the OP.
The other non programing option is to just import the private keys into core and then do a re-scan it's just using a command prompt and doing an importprivkey copy paste over and over as discussed.
Will take longer and you need a synced node but it's not impossible.
-Dave