You can accrue seeds from Raid events by saving all the ones you get from tickets and medallion rewards rather than burning them. The more often you MVP, the more tickets you'll get, and the more efficiently you'll be able to earn seeds. The medallion rewards have diminishing returns, so you can get most of the rewards just by using passive regen. Only those who try to hardcore farm every Raid event will ever have a problem with seeds. If you stick to 1-2 elements you should have no issue, even if you don't play often. Otherwise, you'll just have to play a lot and use that passive regen to its fullest. That's the price you pay for not focusing.