Thank you for this illuminating post.
No, I haven't considered a 61... I never even thought about a 61 (or a 75) as an alternative to the 51.
How does the 51 and 61 compare and contrast in your opinion?
I have found the 61 a little more modern in feel and the nibs to be more consistent, although my sample size is only small. I have had 5 or 6 P51s and maybe 3 or 4 P61s, but the 61s have been nicer to my mind. My current 61 is a solid 18K gold overlaid pen and is absolutely amazing. The nib is a fine, and properly fine, yet still perfectly smooth and reliable.
The main complaint people have with 61s is the complex capiliary system, which admittedly has been the filling systems in my pens. Perhaps I have been lucky, but I have never had a problem with them. Cleaning them on the other hand - just forget it. It takes ages and ages to properly remove old ink. For me, it's not been a problem because I have too many pens to be bothered about changing inks in pens, but if it's a EDC, it would be a concern.
Later 61s have cartridge/converter fillers like a 45/75 and are going to be easier to manage. Once again, 61s seem to be cheaper than 51s at least in the US. The examples out of the UK are generally in much better condition - many seem to have been almost never used and cost from 20-50 pounds for a good example. I'd say the lack of hype compared to the 51 means you get a higher quality pen for the price, and it does have a more modern feel in the hand.
I never got on with the 75. Heresy to some, but it is too slim for my hand. The 45 though with a 14K gold nib is a very cheap and reliable pen, and may be a good entry to Parker to see if you like the general feel. Not too different in writing quality to either 51 or 61, but with a little more give in the nib. Try to get a flighter, as the all-plastic models are exceedingly light weight.
I'm probably confusing you more than helping, but if you have found the Lamy not to your liking, you have a lot of great options with 1950-1970s Parkers, almost all of which are excellent writers.