Well just my 2 cents here but… a dealership will always spot future work for them to do under your hood. However… Your current 19 year old valve cover gaskets have probably been “weeping” for years at this point. Gaskets that are almost 2 decades old dont owe anyone anything. I would simply have an independent shop change them for you, not the dealership and not for anything near the 600+ estimate you received.
I’m sure they are leaking and have been for some time now. They shouldn’t have been disturbed during the timing belt service either so… They are old…just replace them…they cost about 40 bucks. So keep that in mind when shopping for estimates.
Thank God I am not at the mercy of garages or shops… I dont know how you people do it honestly…maybe one day when Im too old to spin wrenches I will find out… Not looking forward to that.