I don't know what the Spinervals test is about, but if it's for training zone setup there should be some instructions on how they expect you to use the info. But based on what you said in your first posting, we know that 1) your theoretical max is a bad prediction, and 2) you have observed a highest value of 198. It may be a decent working starting point. I agree that "if it felt easy it probably wasn't your real max," but it's better than your theoretical value. If you can get time in a sports lab, tri shop, or other venue where they offer coaching and will help you at least get some zones properly set up, do that. If not, set up some training zones using 198 as a rational starting point, and see how it goes. If you want to set up zones as a basis for pacing yourself, it should be ok. It's just a way to get started. You should get or do a proper test.

I think there's some value in putting oneself on a disciplined path, even if it's still necessary to fine-tune the training zones. You have a basis for doing that much.

Another idea is perhaps to just add 5 (just my guess) and assume 203 as an initial estimate of your max. It will get you a somewhat more strenuous set of zones that is closer to your reality. I wouldn't suggest any guesswork in a more aggressive direction than that.

But keep in mind I'm not a coach nor do I stay in Holiday Inns. I just think this approach makes sense in a basic way.