I for example set the score increase to 4 for messages coming from these free services, and my SpamAssassin filter starts flagging e-mail as suspicious at 5, and as spam at 8.

Because the negative scores from these DKIM, SPF and DMARC policies will deduct points from the 4, e-mails will only get flagged as suspicious if they contain obvious spammer techniques such as all-caps subjects, or mention viagra, or money.