Sourdough: 2 questions

day 1 mix 30 grams of yogurth + 20 gr of rye flour; stir every 8-12 hours

day 2 add 30 grams of yogurth + 20 gr of rye flour; stir every 8-12 hours. You will surely see the first signs of fermentation.

day 3, guess! add 30 grams of yogurth + 20 gr of rye flour. Most likely the starter will rise furiously and overflow the cup. After 24 hours add enough flour to make the starter stiffer, wait 1-2 hours and store in the refrigerator until you need it.

No need to refresh regularly, it's immortable! I refresh my starter only when it's about to finish. No waste, either: nothing is discarded.

Or skip the yogurt/apples/vinegar/citrus/randomacidicliquid and just add water to flour.

You can limit waste by keeping very small amounts, and/or utilising the fridge.

It's been a long time.  Welcome back!

Mini

Make a smaller starter if you aren't using it on a daily basis.  One-half cup total  So, you throw out less, and, if you refrigerate, you can get away with replenishing it maybe twice or even once a week.  Not so much waste, not so costly.

Larry

I now keep keep 90 grams of SD starter in the fridge and feed it once a week even if I am baking with it.  Feed 15 G of starter with 35 g of flour and 30 g of water.  Let it sit for 2 hours or so, until it increases in volume 50%, then add the remaining 10 g of flour and refrigerate.  I hardly ever throw any away.  I use 33% each rye, WW and AP for feed.  I then build the levain toward what ever I am baking by using 20 g of starter  and building it to what ever amount I need over (3) 4 hour builds where a progressive amount of flour and water is added to it with the last build establishing the final hydration of the levain.

... yes, though not exactly that recipe, but many, many variations thereof.

Question 2: I think to answer that best, need to ask you a question - how often do you want (or think you want) to bake sourdough bread?

If your answer is only now and then, I would go with the yoghurt & rye recipe given above. And even consider making starter to order. It might be more economical than keeping one going for infrequent use.  Not so long ago,  I fooled around with a starter based on live yoghurt and stoneground rye and using that baked on the starter's third day of existence. (You'll need warm temperatures to achieve that speed of development). Because it's so quick to make and so active so soon, and because I don't want to bake sourdough more than once a fortnight - I don't bother keeping any starter at all.  Just run up a fresh batch 3 days before I bake.

All at Sea

On your second question, I say why do you need to use a premium-priced flour for a sourdough starter?  You're making a starter only to carry flavor and yeast into your dough.  In my kitchen, I use the least expensive generic unbleached flour I can find.  I have not noticed that the function or taste of my sourdough starter suffers at all.  The price of 2 ten pound bags of white flour at Costco's is fairly low, so it's worth it if available to you and you've somewhere cool to store it.

Oh, yes, I've had good luck getting low prices on 4 gm protein flours (even King Arthur Bread Flour) at Wegman's in Pennsylvania.  Maybe you know someone who lives down there?