Core strength help

Have a look at the proposed free exercises here - I am not advertising her program but she does speak sense.

What's wrong with using the word core strength?

Many people do sit-ups wrong by not using their deeper muscles and they can actually do rather damaging to women who have been pregnant (regardless of how they delivered).
It took at specialist Women's Health physiotherapist point out to me last year that the pelvic floor muscles and the deep abdominal muscles are connected (as are the back muscles); it is really all one system.

Pilates and yoga can be great to address these issues.
Also get doing yer pelvic floor exercises every day.
Get in the habit of sitting down on a chair slowly and rising fast (without using arms), do squats while bracing your pelvic floor and pulling your belly button towards your spine, also do this when walking.

How often do you run & how far each time?

I fucking hate that word "core" but I know OP didn't ask that

I don't understand how OP can do a plank but can't do a sit up. Maybe not doing plank right. It's very specific what you do with your bottom, back, etc. I am hopeless at planks & have back problems from bad posture (so my stomach muscles are trash, really) but I can easily enough do situps/crunches etc. Or else those don't things link together that OP tried to link together.

Fellow runners tell me that resistance exercises as in gym bunny activities, pushing weights are what one does to be able to run a lot without injury. Not as simple as planks.

Have a look at alignment, to make sure that your efforts are working well together

https://nutritiousmovement.com/about/

This has made such a difference to me, I used to make so much effort and always ended up damaging something. Hope you find what you need.

Pilates is good but there are a few moves that will compromise a weakened core / diastasis. You should search for post-natal pilates rather than general. Also if you google for pilates moves unsafe to do with diastasis it might help. If you can see a physio I would do that first though, especially as you know you have DR. The gap doesn't need to be big to cause problems, but if it's only 2.5cm it's easier to close or improve than if it were much larger. You might need to hold off on high impact workouts for a little while while you focus on improving your core but as you already go to a gym it should be easy to switch to lower impact activities - I would ask the physio and they will be able to advise based on your symptoms and their assessment of you.

Yes very worth seeing a women's health specialist physio even years later.

If you have diastasis or weakened muscles from pregnancy / birth planks are not going to help until you've addressed the underlying weakness. They are quite an advanced exercise and could make things worse - all that pressure pushing out is having the opposite effect to the one you are trying to achieve. I would definitely recommend seeing someone but make sure they specialise in post-natal / diastasis rehab.

Definitely see a physio but look for one that really does specialise in women's health (and doesn't just say they do). Also Pilates is ideal for his sort of thing. I would suggest getting guidance on any core work so that you don't do the wrong thing and make it worse