@silkweed
Assuming that the first pictures are when you decanted from " the bucket " into the bottles there's plenty of yeast. But your Specific gravity suggests they have used up all of their food so when bottled couldn't make any gas.
You didn't add extra sugar at first bottling I don't think.
Given that the yeast has fermented down to 0.990 it's pretty alcohol tolerant.
Try this free calculator
https://www.brewersfriend.com/beer-priming-calculator/
Normally champagne is about 4 vols plus.
Your bottles look nice but given their square shape did they have something under pressure when you bought them or are they pressure safe.
I use champagne bottles as they are stronger and thicker than normal beer bottles. If you fill a weak bottle with sugar and yeast pressure builds up and the bottle bursts ( bottle bombs are dangerous ). If they are all together you can get a chain reaction.
Once you have decided how much dry " sugar " is needed, say you have 20 bottles boil just over 200 ml of water. Remove from heat and add the sugar. Let it dissolve and cool. Then 10 ml of sugar syrup per bottle with a syringe ( all sanitised of course ) then put caps back on no need really to shake them and then leave for at least 2 weeks in about 20 celsius. Then cool a bottle in fridge and test it.
I made my first elderflower fizz last year having said I was going to make it for years and finally did. Two batches and fantastic.
I followed an internet recipe and those who have also tasted it thought it was great. It was tricky to find the elderflower down here in New Zealand and it didn't smell as vibrant as I remember in the UK but certainly did in the glass and whilst making it.
If the bottle hasn't fizzed up after 2 weeks I'd leave another week and retest. You should have some signs of activity though.
If no activity I'd repeat the process but no sugar just use boiled and cooled water mixed with champagne yeast and a bit of wine nutrient.
I fermented one of my batches with kveik yeast followed by champagne and the other with champagne yeast alone.
Will get a picture of it bottled and upload it later.