Hi
@bernina! Long time no see!
What's your 5/24 status like these days? Chase changed the CSP and CSR to only one signup bonus every 48 months, so depending on how long ago you received your last Sapphire bonus that's not an option. If you're under 5/24, and open to signing up for business cards, the Chase Ink Business cards (the CIC, CIU, and especially the CIP) are an option to build up some UR points, which can be transferred to United miles if you find a good redemption. If you're under 5/24, Chase also put out a new signup offer on the United Explorer card yesterday, but it's a tiered bonus that requires you to spend $8k MSR to earn 60k United miles.
As for Delta, Amex is currently running increased offers on their Delta personal and business credit cards until 5/1/19:
Increased American Express Delta Offers Now Available
Increased American Express Business Delta Offers Now Available.
I recall you have a Platinum card. You can use the Platinum card to generate a referral link, open that link in a new browser or incognito window, click through all the different cards with a referral offer, and find the Delta personal or business card with the increased signup up offer you want to apply for. You should receive the increased signup bonus and a 15k MR referral bonus on the Platinum for the self-referral.
However, as with all things Amex, you'll only qualify for the signup bonus if you don't have and never had that card product, and if you don't get the bonus denial pop up that warns you that you're ineligible for the bonus because of your "history." If you don't want new cards counting against 5/24, then you should also stick to business cards.