didn't want your post to go unanswered, but try not to panic. I'm not an expert, but have done a bit of training as a peer supporter. hopefully tiktok or hunkermunker will see this and be along to reassure you soon.
firstly, six weeks is a time of growth spurt and babies feed more than normal, so it's a common time for boobs to feel empty. It's only a feeling though; the more a baby feeds and the more you express, the more your body will produce.
Also, if your feeding is reasonably well established (usually from around 6-8 weeks) then your body is working on the supply and demand basis that you are aiming for with BFing. Is your DD feeding well and seeming to be satisfied afterwards? She would soon let you know if she wasn't.
As for feeling bruised and battered, the recommendation, certainly in the avent manual pump, is to start off gradually (for a few minutes at a time and build up). It is quite a brutal action, not gentle like your baby suckling.
evenings are often a time when your suply dips a little as well. Expressing first thing in the morning is usually more productive.
Try not to be disheartened; it sounds like you have been doing everything right and that your body is doing what it should. Perhaps lay off the expressing for a few days and then start again gradually, and put your baby to the breast as often as possible in the meantime.
there is a lot of good advice on hunkermunker's blog and kellymom
hope this helps and that someone more confident at giving advice is along shortly.