Is 5 Reps Enough?

Any starting point that creates useful challenge, but doesn't cause injury, is a good starting point.

Your plan is fine. (I'd agree about doing preparatory exercises for anything that's threatening injury. You can build up a little bit via prep exercises, then go back to the more advanced exercise. Some good suggestions in the post above; you can also look for exercises on YouTube from physical therapists who have actual credentials/certifications. (Physical therapists are the experts in overcoming limitations, IMO.)

After a while, if you stick with it, and avoid injury, you'll find that what you're doing is getting kinda easy, not really a challenge anymore. When that happens, increase the challenge: Do more reps, or more sets, or change to a manageable but more challenging exercise, or - only if you can and still leave a recovery day between strength challenge to the same muscle groups - increase frequency.

Things getting easier is a sign of increasing strength/fitness. Increasing (something) to keep a manageable challenge going, that's how to continue increasing fitness.

We start wherever we are, wherever that initial manageable challenge is. Exactly where that is (in terms of exercises, reps, sets, etc.) is a question of good or bad. I've said it so many times here on MFP that I'm sure others are sick of it, but I think the wisest sign in the whole world is one we see all over the place: It says "you are here". Here is the only place from which we can take the next steps toward self-improvement. It's the steps that matter, not the starting place.

P.S. Planks in particular can be an extra challenge when we're still carrying a good bit of extra weight, if that's what your current weight is for you. Basically, you're asking your core structure - which has a bunch of small muscles in it, as well as a few big ones - to support all of our weight in suspension between hands and feet. That can be Too Much.

I agree that Russian Twists are iffy. The Bird Dogs that Snifterpug suggests can be good, or check out Bob & Brad on YouTube (Physical Therapists) who have various videos with plank alternatives, to strengthen core. Search "Bob & Brad Planks" within YouTube, you'll find them. (I don't want to link them here because I find the in-line videos annoying, personally.)

The question is, is it enough for you right now?

The way strength training works is that you increase your work as you get stronger. So 5 reps or even 1 rep for some people may be all you can do today, but that doesn’t mean it’s all you should keep doing as you go forward. Don’t be afraid to start where you are, even if it doesn’t seem like much, but also don’t let that stop you from adding more. So, maybe six reps, then seven, and so on, until you add a second set, and then you get sick of doing a million body weight reps so you add some weights, and in a year you are doing more than you ever thought you could.

All of this will get easier as you lose weight, too. Jumping jacks and other high impact activities in particular will get easier.

BTW I am not a fan of side planks. I permanently tore up my shoulder doing them when I was obese. Now that I am normal weight and can do them easily, I don’t think they are a good exercise for working the muscles they are supposed to target in any case.