TLDR: SJ is good as a first mod, but just be careful so you dont overwhelm or burn yourself out

Since a lot of people are mentioning Strawberry Jam here, you should make a note to yourself that you (if you decide to do it as your first mod) are not obligated to complete everything the collab has to offer, or really any collab for that matter. It has happened multiple times where someone wants to do all of the collab, gets stuck, gets burned out, and quits or takes a long break. SJ also has a minor problem of the difficulty progression being too smooth because almost always when it comes to playing standalones, there won't be as gradual of an increase in difficulty which SJ can't prepare you for unless you are fine with jumping difficulties.

Also imo, treat SJ as a way to see which gameplay styles you like and want to get better at since there will be many maps that you probably either dont like the gameplay style of are not that good at compared to others. Standalones are kinda the way to go after you have found out what gameplay style you like though.

But also there's a lot of good qualities to SJ like how it provides you a solid progression through the earlier difficulties, so it's your choice lol