The most important is the drainage. They can tolerate some richness, as long as the soil drains well. Sandy soil with added manure or compost is OK (I add it around roses but not directly around lavenders). Clay soil or soil that gets waterlogged in winter would be a problem. But yes, if you have the perfect soil for roses, it isn't good for lavenders. My soil is much better for lavenders than for roses, sadly.
The soil under my window is quite poor and shallow which is why I put it there, the other lavenders on edge of new patio are in a similar soil as I didn’t enrich the soil and it’s quite gravelly. That’s why I grow some in pots, at least you can place the pots without worrying about the soil being too rich.
Life's tragedy is that we get old too soon and wise too late.
Best plan of action I think is just to buy one plant from the local GC and see how it fares over the coming summer and beyond before I buy the number I would need.
As an alternative to lavender if you have richer soil, why not try Nepeta? Similar colour palette and bushy style but more tolerant of rich soil (and even a bit of shade)? There are some lower-growing varieties
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But yes, if you have the perfect soil for roses, it isn't good for lavenders. My soil is much better for lavenders than for roses, sadly.