Did not accept by "congenial", which is a meaning in my dictionary. https://www.spanishdict.com/translate/simp%C3%A1tico
I agree that it is a little fast for a beginner, (which we both are) but I also recognize that it is also the regular speed of speech and if you want to learn to understand, you also have to learn to process quickly. So keep at it, and so will I, and eventually we will get fluent.
Tener is the infinitive form, so it's used when you're saying "I want to have a nice boss." ("Yo quiero tener un jefe simpatico.") Tengo is the first person present tense indicative form, which would be used in a case like "I have a nice boss." ("Yo tengo un jefo simpatico.") Basically, they are different "forms" of the same word.
The problem is that "nice" is a very broad adjective in English, whereas "simpatico" is more specific in Spanish: Think "nice" in the terms of "likeable, friendly, kind" when applied to people.
"Nice" when referring to a house would more likely be commenting on its appearance, so you'd use something like "bonita / linda casa".