I had exactly the same issue as you, I flew to Mallorca (I live in Spain) and visited a bike fit centre and sports physio at the same time.
For me the bike fit solved 90% of the problem, I now only get the occasional cramp but no tendon issues and extending my mileage up to 120km per ride without problems. The main change was saddle height, I thought I had it set correctly but it had to go up by 2.5cm (gradually). This better leg extension stops dropping of the heel, plus I needed wedges on the cleats so the knees tracked better.
I also have a set of exercises that help, but TBH I haven't had to do them. I can send these to you if you PM me an email address....