iShares Core € Corp Bond vs iShares € High Yield Corp Bond

They track different indices: iShares Core € Corp Bond follows Bloomberg Euro Corporate and iShares € High Yield Corp Bond follows Markit iBoxx EUR Liquid High Yield. They also differ in cost (0.09% vs 0.50% TER), replication (physical vs physical) and domicile (Ireland vs Ireland).

Nobody can know that: they track different indices, so their returns will diverge, and past index performance is not a prediction. What you can control today is the cost, the tax structure and whether the index actually covers what you want exposure to.

iShares Core € Corp Bond has the lower annual fee. Applying the same 7% gross return to both with €300/month, that fee gap alone is worth about €7,548 after 20 years — that part is arithmetic, not a forecast.

Both are distributing, so on this point there is no difference between them.

Yes. Unlike two funds tracking the identical index, these give you genuinely different exposure, so holding both is not automatically redundant — just be aware you may end up overlapping on the largest companies and doubling your rebalancing work.