238km 5500 altitude - help me at least try

I agree that smooth z2 would be a very comfortable approach.
At Ötztaler this is very difficult. There is only uphill or downhill within 4 alpine passes (Timmelsjoch is one of the highest of the alps).
The uphill sections are very long (especially the last) with very steep sections (16+%).
So it will be hard to maintain z2 with an ftp of 210.
But the downhill sections are long as well. At least there are long opportunities for coasting.

You’re doing the Ötztaler Radmarathon!!! I’m jealous! Have you seen this pacing calculator? You feed in your FTP and system weight and it gives you watt targets for the different parts Ötztaler Radmarathon – Realistische Zielzeit. Eine Prognose. Meine Strategie!

My first big mountain events were done at (educated guess) ~260-270W at 93kg (~2.8-3.0 W/kg). Gearing was subcompact 50x34 and 11-32 cassette. 8 hours climbing at ~65rpm and no problems. A lower gear would have been nice, but I didn’t see it as a problem.