Note that SkyMiles Gold requires 10k MQD, not 12k. Also worth noting that you do rollover excess XP when you qualify for Silver, and that Flying Blue does soft landings (even if you don't hit 100 XP in a year as Gold, you won't drop below Silver; if you don't hit 180 XP in a Platinum year, you will keep Gold for another year). As you have seen, BOS-AMS/CDG is special cased in Flying Blue to earn more XP than it should by rule (LHR/EDI/DUB does not get such a benefit)

If buying First Class domestically, I'm of the opinion that it's generally better to go for Flying Blue status (especially in view of the easier path to lifetime SkyTeam Elite+ by being Flying Blue Platinum for 10 years (a bit harder with the changes to XP rollover coming over the next 9-21 months, but still practical)). Flying Blue Gold will get Sky Priority boarding on DL: I believe Flying Blue Silver in Main Cabin will board in Main 1.

If going for FB status, it's highly advised to get the BofA Flying Blue credit card ($100-ish annual fee): it typically offers 60-100 XP as a sign-up bonus (analogous to the DLAX Gold offering a 3-5k MQD signup bonus...), though doesn't by itself confer any benefits for holding it and 20-60 XP every year (20 XP for renewing, 40 XP for spending $15k in the previous year). The card earns 1.5 FB miles on unbonused spend (so comparable to a 2% cashback card; if willing to book close-in/at slow times of year, it's possible to get better than 2.5 cpm on AFKL J awards (last year's $10k roundtrip BOS-CDG-BER (late booking for me) was 600k-ish FB miles to bring my wife and son along with me)).

To make getting FB status even easier, note that on AFKL metal, they sell a "sustainable aviation fuel" add-on that lets you effectively buy XP for no more than 10 euro per XP (for a BOS-CDG-India roundtrip, the "full dispensation" offer will probably be about $3k and give something like 300 XP extra... if buying for the roundtrip, the XP typically credit after flying the outbound and before the return (so you could board the return flight with Gold status and 80 or so XP before getting the return trip credit)).

Also: my boarding passes on DL (including in the app) will typically say "Platinum Medallion" (and then say AF/Flying Blue elsewhere). While automated systems will likely not treat you as a DL elite, this may trick gate agents etc.