I had same problem in my wrist after a break.
I had 2 cortisone injections over 12 months.
Problem has since dissipated some what.

We do so many steroid injections at work under imaging guidance.
They are great and Ive had one in my shoulder for biceps tendonitis.
A steroid/cortisone injection is a powerful anti inflammatory that is directed around the tendon. Tendonitis basically means inflamed tendon - you get fluid building up around the tendon and that causes pain. A steroid injection aims to clean up that fluid. Some people can get a cure from these (my tendonitis never returned), some people only get a bandaid fix (i.e. they have a mechanical or anatomical problem thats causing the issue in the first place and may need to keep having them in the future or have the problem managed by a specialist), and for some the injection doesn't work at all.
Its a very quick procedure and and you really have nothing to lose. There is always a risk of infection as you are breaking the skin with a needle, but there is always a risk with everything else in life and of course everything they use is sterile, employing a sterile technique. At worst, it just doesn't work and at best you can go on and enjoy pain free kiting.
Hope this helps.

I had patella tendonidis on my left knee, kite surfing related. Bloody painful, even at rest (could hardly sleep).

Cortison injection, anti inflammatory pills but mostly lots of ice packs. If you feel it flaring up, ice is my advice. Apply for an hour or so twice a day.

If it persists, you could consider wake board style booties, take the strain off your toes...

Good luck mate.

en.m.wikipedia.org/wiki/Methylsulfonylmethane

Do a.search for it, there are always the nay Sayers but if you think it does the job you are halfway there.

There is an Ozzie brand that I buy, www.msmaustralia.com

I.always like to share this type of info to get a better consensus on what works and what doesn't . I like to experiment but I.think the MSM really does improve the recovery time

Try some Dakine Raptor heel bungies,they keep you locked in so no need to put excessive strain on your toes

Before injections get some xrays and a ultra-sound and get a specialists to check. I had similar probs but around the ankle joint. Turns out it was a bone spur from footy days. Day surgery, couple months later and I can now run on it for the first time in years. Well do the cliff young shuffle anyhow...doesn't hurt when kiting anymore either. But yes strap and technique adjustment is crucial as I had to sort this out to keep kiting for 5 years with a bone spur.