Objectively speaking, it’s a mortal sin against the marital union if a man stimulates a woman (or woman stimulates a man) outside of the marital act, regardless of whether she/he climaxes.
However, a husband can stimulate a woman as part of foreplay before the marital act but not up to climax. If the woman has not achieved orgasm within the marital act, the man can stimulate her to climax AFTER his climax within the marital act.
So objectively speaking, what you described would be a mortal sin, so it’s grave matter. However, from what you described, you and your husband didn’t know or weren’t sure that it was a mortal sin, so you did this without full knowledge, making it a venial sin. The three conditions for a mortal sin are 1) grave matter, 2) full knowledge and 3) full consent. You didn’t have full knowledge.
If the two of you know a good priest who is knowledgable of moral theology (and just doesn’t tell you that you can do whatever feels good because God understands), it would be good for you all to meet to get this ironed out so you know what actually constitutes mortal/venial sin within the context of the marital relationship. This is just so you two can better communicate and make decisions during times of temptation. This article might be helpful to you and your husband as well. The section most directly related to your question would be covered under “Other Violations to Chastity.”