I don't think dropping the server-sig-algs packet gives any additional downgrade capability? If a server is going to accept a rsa sha1 signature it will accept it regardless of whether the client receives the server-sig-algs or not. Performing a rsa sha1 signature doesn't cause any exposure for a client (that I know of?).

server-sig-algs was only added for compatibility, so new clients wouldn't get kicked off old servers that don't accept rsa-sha2 pubkey auth packets. https://www.rfc-editor.org/rfc/rfc8308#page-7