Hi and welcome.
Your garden is probably slightly longer than ours and part of ours is not much wider than yours as we've an extended garage and shed ocupying part of it and then a summerhouse at the bottom.
It's a question of how much work you are prepared to put into it, whether you want to just tidy it up or start from scratch. A complete re-vamp even if you do it yourself as I did ours, will cost a lot of money, but you can spread it over several years.
There seems to be an awful lot of concrete in the garden. We have probably as much in the way of hard surfaces, but most of it is outside our French windows.
What you need is a long term plan and work towards it. Not trying to do everything at once, but starting with the bit closest to the house as getting that right will encourage you to do the next bit. I would get rid of any "straight lines," as curves give more interest.
I'd keep the tree but it needs a drastic prune.
We have a tiny front garden, with a big acer tree, but I prune it every year to keep a sense of proportion. It's about 16ft tall.
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I did all the hard landscaping of ours in the mid eighties over the course of two years and then "let it grow." All we've done since, is rung the changes as regards plants.
The plan did include a 5ft deep 3000 gallon koi pool.
Apart from our patio, the rest of the garden is all curves. I have to admit we've spent thousands on it over the years, but mostly just on plants.
Even when I built ther pool in 1986, I had in the back of my mind what I would do if we ever decided to to get rid of it. We did just that last year when it developed a serious leak and had it filled in and paved over.
Here's a tour, it may give you some ideas. Anything is possible in a small garden.