The German sentence seems rather loosely worded - it is the rent itself that is increased incrementally, not the yield from the rent. That is probably why the two words together (Mietrendite, valorisieren) are not found on Google.

On the other alternatives:
"indexing": the examples I have found do not relate increases to the cost of living index, they are either based on a flat rate percentage or the general concept of an increase, as in "das Pflegegeld ..., das seit 1998 nicht valorisiert worden ist"
(see www.wien.gv.at/mdb/gr/2003/gr-032-w-2003-09-24-027.htm)
"subsidised": a price increase is not a subsidy.
"valorise/valorize": the word exists in English and seems to be used in various meanings (perhaps nobody quite knows what it really means). Oxford says "give value to" or "raise prices by artificial means, usually by government action".