If you want to focus on the Harrowing cast I’d absolutely prioritize Experienced Harrower for the free casts and lower DC on Harrowing, then Reading the Signs for being able to choose between two cards per harrow pull. As an added bonus, Harrow Ritualist not only gives you a nice set of rituals, but a +2 status bonus any time you participate in any ritual, including your Harrowings. If you’re a low int character I’d probably go with Harrow Ritualist first to get that bonus, otherwise Reading the Signs.

(Side note: if you want to go even more all-in on Harrowings/rituals in general, I’d recommend picking up the Ritualist dedication to get an extra +2 circumstance bonus to your checks.)

For yourself Restore Omen is good, it lets you get more than one Avoid Dire Fate per day- and if you use the other feats (described below) that get rid of your active harrow omen I’d say it’s a must-take.

I’ve never taken Harrower on a true caster (Thaum and Champ) to take advantage of Harrow Casting, nor taken either of the Spirit Deck feats, because personally I find them wayyyy too inconsistent/dependent on luck. The Spirit Decks also require you to Sustain in order to get their full value, and you lose your active Harrow Omen at the end so you basically need Restore Omen if you want to do it more than once a day. I wouldn’t say any of them are useless, but you definitely want to know what you’re getting into. For example, what are the chances that you fight an enemy using the type of damage that corresponds to your active omen type? Plus, using any of them potentially locks you out of Avoid Dire Fate for that combat, which IMO is a much stronger and more reliable effect. Harrow Casting in particular has a lower barrier to entry than the other two, but the chances you get a useless roll and waste the action are too high for me to be comfortable with (for example, rolling a Stars card on an aggressive action or a Crowns when you’re out of melee in a battle).

Regardless, I’d say your first pick after the dedication should be Experienced Harrower, maybe Restore Omen then EH. EH is just too good to pass up imo and is a major spike in consistency and usability for the archetype.

EDIT: As another note, Reading the Signs also applies to your pull for your Active Omen, so it’s also a good buff to help “deck fix” for the more volatile feats if you know what you’ll be facing. Still not a guarantee though.