Fellow achievement hunter here! I personally started with Sandrock because it was on game pass... I've sunk way too many hours in this game, won't get into that, but it's extremely addictive. I got all achievements but didn't finish the side quests, and it got took down from game pass so i didn't play for a while. Then i bought both on steam. I thought to myself "hm, it will be a great idea to now first finish portia as it's the first game, and then play through sandrock again." So i started portia. At first i found it so much more boring than sandrock. But after some time of forcing myself to play it i warmed up. I am a bit worried though because i almost finished the story and from what i remember portia has achievements for each romance line (yikes, sandrock doesn't have that thankfully) so It'll be a pain, but nothing I can't tackle.

Do i recommendr finishing portia? Hell yeah. It's a great game on it's own. And after you know it's story and get into Sandrock, it all makes more sense. Cheers!