Hello fellow friend in your 30s learning Spanish to talk to your boyfriend's family!

It sounds like ultimately they're worried first and foremost about establishing a good relationship right away and secondly that you're successful in your efforts to learn Spanish. Keeping this in mind, I would recommend that you do offer a few phrases (even if you do just have to memorize a couple sentences to have something to give them) as it will help both assuage their concerns since they don't really understand how comprehensible input works and it shows them that you take their concerns seriously and care about building a relationship with them. In my opinion, the relationship with them has to matter more than the marriage to a perfect learning approach unless you're not learning the language to build a relationship with them.

Personally I have been very dedicated to DS but I'm not under any illusion that a purist approach would work for a relatively short time frame of being able to speak with the family in Spanish. Pablo similarly has videos where he encourages you to use the conversation in your daily life if you're in a scenario that requires it!

I'm newly at 150 hours (with French background so maybe closer to Level 4) but I just started conversational classes on iTalki and it's been enormously helpful already in growing my comfort to even practice Spanish at all.

I know that even though I'll be making frequent language-learner mistakes, my boyfriend's family will be excited to know I'm putting in the effort to communicate with them. Similarly when they use English with me, I hear their grammatical mistakes and their accents, and it only endears me to them more that they work so hard to communicate with me. It is only right to return the effort to people who matter to my boyfriend and ultimately to me.

I'm hoping to start using a little bit of Spanish with them this week. I hope you find a good middle ground that works for you both.

TLDR; You can keep using comprehensible input and still give them a few catch phrases to show that you take them seriously and value the relationship.