I never come across such KYC verification where they need income proof. You are not opening the bank account where they need you the give the income proof  They are either not willing to approve your KYC or you wanted to gambling with huge amount of money. Level 1 KYC is enough for most gamblers and it will never ask you the funds proof.

Also balance in your Electrum wallet isn't an income proof. It is just to show that you have that much money, Income proofs are usually the salary statements or business profit/loss sheets that show your running income. Bank statements / wallet balances are not income statements.