Is there a solution to falling royalties?

As the weeks go by and my sales and page reads continue at 30% of what they were last year, I am forced to take stock of my options.

£2.99/$3.99 Amazon.
"myBook.to/WW2saga"

I threw everything I could think of at the publication of the first book in my Ellen's War series, Blue Skies & Tiger Moths, but it has still failed to gain traction with my readers despite having only four and five star reviews.
I am participating in a promotion to coincide with the anniversary of Dunkirk in July so am holding fire on doing anything else myself until after that.
I did a Book Bub advert, a Thunderclap, Book Viral, a Twitter campaign and all made no difference at all. A lot of time and money invested for no discernible return.
I am not the only writer seeing this unpleasant downturn – I've been contacted by several others who are in a similar position and, like me, are not sure how to improve things.
It is ironic that my writing is going better than it has for years when sales are so poor. I had got used to having a lot of disposable income and will now have to reconcile myself to having so much less, whilst doing so much more work.
As I have mentioned in a previous blog I don't think it is anything to do with the quality of my books it is that the amount of books available has more than doubled over the past four years whilst the amount of readers has not grown in tandem with this increase.
I'm just going to hang on in there in the hope that writing good books and publishing frequently will be enough for me to maintain the position I have at the moment. My rank both in sales and author on Amazon has remained about the same, which suggests that readers are spreading their buying over more authors. 305 of what I was earning is still reasonable - imagine how bad it is for those who were not in the same fortunate position as I was?
I published the third book in my Regency series, The Duke's Alliance, An Unconventional Bride, in February and this too has failed to become a bestseller which the previous two did. It  sold well enough to cover my costs and make me a reasonable profit but never really took off.
A close friend, also a Regency writer, someone who is on all platforms and not Amazon only as I am, has seen a huge surge in sales and has been a bestseller for several weeks with one of them. I think maybe it is time to try uploading some books on all the platforms.

£1.99/£2.99 Amazon

If anyone has any suggestions as to how writers in the same position as I am can claim back the lost ground then I would be delighted to hear them.
I am wondering if setting up a writers consortium, and then all publishing under the same label, might be the answer. By so doing those involved would be eligible for Amazon promotions and so on. What do you think? Is this a viable option?
Everyone says that growing one's reader subscription list is essential – but mine remains stubbornly around 800+ and every time I send out a newsletter I lose a few, which I slowly clawback.
I've decided I'm not going to go down the Facebook/Amazon/book bub adds route as they are expensive and time consuming and don't necessarily work.
So far this year I have written the two books already mentioned, the third book in the, Nightingale Chronicles, Better Bend Than Break, which will be out at the end of June. I have also written the fourth book in the Duke's Alliance series, An Accommodating Husband, and am 1/3 of the way through my contribution to Regency Romantics box set for the winter. I shall then write the second book in Ellen's War and the penultimate book in my Regency series. If there is time I shall write my spring contribution the Regency Romantics and start the fourth and final book in The Nightingale Chronicles.
As you can see I work very hard. I still consider myself fortunate as I love what I am doing and I get up every day eager to get on with the work in progress.
Fenella J MIller