If they want to hire you as CTO but are reluctant to give you shares, that should be a meaningful red flag. Bonuses for a rewrite being successful are pretty questionable territory - they'd need to put hard metrics in place to define what success means in order for you to get your bonus, and that's tough. Commercial success? You could do an amazing job rewriting the site and not see that due to aspects of the business that are completely beyond your control as CTO. Technical criteria, like, I dunno, pageload speed? They shouldn't want to do anything like that because they're weird and can be gamed.
In terms of the percent of the company you should ask for, there's not enough information in this post to give a useful answer. How's the salary relative to the market? What's your experience? What do they anticipate the impact of the rewrite being? How much business do they expect country B to drive vs. country A, which is already established? If it's a market leader but only valued at €15-20M, then it sounds like it's in a niche category and isn't likely to see explosive growth (unless that's coming from country B and future countries, in which case your work will be driving most of the growth, and you should be rewarded accordingly).
Your description of the work makes it sound like you're being hired more as an engineer than a CTO - they've already decided what the strategic direction is and what work needs to be done, and you're just there to write code. The lack of willingness to give equity to the second- or third-most senior position in the company adds to that impression.
You should figure out what you could get in a more traditional engineering role, then demand some form of compensation that will have substantial upside to that in the event of success (with the amount of upside being directly proportional to how much less you'll make vs. working as an engineer if the company is not successful).