I'd probably say don't do Tangier. I myself am probably not going to go, and I'll even be in Malaga, Spain for a few days (which is much closer to Tangier, compared to Seville, if you use Google or Apple Maps). I'm currently leaning towards just staying in Spain. I talked with my travel agent and she doesn't know anything about the Tangier, Morocco tours + I heard the ferry ride can be quite a pain (customs is quite a lag on the ferry); from Spain (Seville in your case) to Tangier.
Though it sounds very fun to enjoy another culture (I'm even down for), besides European culture, that would be a very long trek on the trains and ferry. the ferry ride even from the southern tip of Spain to Tangier, Morocco is like 4-6 hours one way! I'm still researching and debating if it's worth the time for me. Again, you do you. I did however get some great advice about travel; "Look at it as an experience. If you don't like travelling from Spain to Morocco, you can tell others about it. It's an adventure. It's only a small bit of your travels" (Fellow Rick Steves person on another forum here).
Not to doom scare people on here (I'm just treading on the cautious side of things, based on the state of the world): The flight situation + security may be longer/tighter too. I know that Morocco is pretty safe-ish, talking to people, friends, & family, but be careful of people in the markets/streets asking for money, pick pockets, etc (read this in Rick Steves Spain/Morocco book). I'm going to check the U.S. embassy consulate in Morocco again/contact them to check if it would be safe for me even (due to the whole U.S./Israel vs. Iran) war situation. I'm sure others on here follow the news, but Iran did try to strike Turkey (European/NATO country even) and Turkey was able to defensively blast the missiles down. Middle East = close-ish to Africa + Russia is helping Iran now with intel against the U.S.& its allies. Morocco is an ally of the U.S., but it's not in the top 25 (from what I looked up), so I'd think about that one too.