The 5x dollar earning on Delta spend with the Delta Amex cards is only a temporary promotion through 2021. The Delta Plat Amex will revert to 3x dollar earning in 2022. The lowest Delta fare on GRR-MSP is $258.80 roundtrip, but you don't earn miles on the taxes from flying (only the base fare). So you would only earn about 1070 miles ($214*5) per roundtrip from flying as the base fare is around $214 roundtrip. However, earn from Delta Amex spend in based on the entire purchase price.

Your math formula (250*5*5) is way off. Earning per trip during the Amex promo would be 5x $258.80 (credit card) plus 5x $214 (flying) = 2364 miles. In 2022, it would be around 1846 miles per trip when Amex earnings reverts to 3x dollar spend.

You'd need to spend at least $12000 or so a year on Delta tickets to justify the additional $150 annual fee of the Delta Platinum Amex card over the Gold card (which earns 2x dollar on Delta tickets). Also, the Delta Gold Amex currently has a first year annual fee waiver. If travel with a companion at times, the Delta Plat Amex might be worth the additional cost due to it's annual companion certificate. If you don't currently have TSA Pre-check, the Plat card also covers the $85 application fee (which must be renewed every 5 years).

FWIW, LAN - MSP flights resume in November and can be somewhat cheaper -- particularly if you can fly on Sat, Tue, and Wed flights.