Hi! I’m looking for some help with my indoor avocado plant please!

Previously, the lower, fully-developed leaves were all green and healthy. However, it is now rapidly going brown on all lower leaves and they are all curling inwards. They have now started to drop off too.

However, about a month ago I pinched off the top few leaves (when it was still healthy) which lead to a couple new branches and lots of healthy green leaves! This new growth is still thriving and as of yet, no brown tips!!

However, I am concerned they will be next. Can anyone please tell me why it is so rapidly loosing leaves and what I can do to save it? Or is this just a natural process? I heard they are naturally thinning and this may be normal, but I’m concerned because its the majority of the older leaves.

It’s in a 21cm pot. The plant is about 56cm high. I lightly fertilised it over a month ago now but when it started going brown I leached a load of water through the pot to bleed out any salts and havent fertilised since, only watering when it dried out. I spray a couple times a day to keep humidity up. Its in a window that gets a bit of full sun, the rest of the time its filtered/shaded light. I live in the UK but its in a room thats always warm - however I do worry about the cold draft from the window sometimes.

Any help is greatly appreciated! I love this plant and really dont want it to die