As long as you plan ahead to accommodate for any required shipping, you can add pretty much tickets from any reputable 3rd party reseller to MDE. If anyone still has non RFID enabled/FP+ compatible ticket stock, I wouldn't think their prices are that good as the new stock has been out for a while now. And even if you receive a voucher, the voucher should still have the information on it needed to link to MDE.
UT typically has the best prices available to the public at large, the 6 day MYW+WP&M+Hopping you mentioned is currently $404.13, ~$21 off the list of $426 (tax included).
However, if you find a good deal on Disney Gift cards, you may be able to do better. That $21 is approx 5% off, and you may be able to get 5% off Disney gift cards at Target (pharmacy coupons, red card, etc) unless they closed that loophole, or your local warehouse club may have them at a discount as well. If you purchase them using a rewards credit card, or as part of a gas savings promotion, you could potentially get another 5-10% in savings (in one form or another).