Good RA4 colour printing is about getting control of all the variables that impact upon what is primarily a process designed for machine processing. I can tell you with certainty because I have done the experiments that you will never get optimum results using Kodak RA4 chemistry at room temperature. The only other brand I have ever used was Agfa when it was still available and the results were pretty much the same so I expect changing chemistry would not help.
You can process the stuff in trays but you need to use a water bath or some other means to get that temp up to 37F/100F if possible or close to it and be able to achieve that same temp every time you process. I know that leads to very short processing times but that's one of the variables that needs to be managed. If I understand correctly you are reusing your chemicals in trays over a 6 month period. If so I'm surprised they remain viable for that time and IMO could not possibly give constant results from batch to batch.
Unsure where you are coming from in trying to match a Velvia 100 print since Velvia is a tranny film and prints from it would have to be made either from an interneg or digitally or on Ilfochrome - none of which produce a result comparable to a RA4 print from a properly exposed and processed colour neg.
I've not found that RA4 materials produce excessively contrasy prints unless the negative itself is very contrasty and that too is an uncommon situation because of the exposure latitude of most colour neg films. Are you processing your own negs? If yes, there could be a problem there. When trying to refine a colour printing setup it is wise to use correctly exposed test negs that have been professionally processed - again it is about controlling the variables.
I have never seen an example of a fuzzy image caused by a lack of resolution in the printing paper. Because colour paper is multi-layer I suppose it is arguable that it must lack resolution compared to say a single layer B&W paper but I've never noticed it. The lens you are using is not the best enlarging lens ever made but it's still pretty good and would be more than up to the task of printing say a 16x20 from a 6x6 neg. Sounds like a focus or paper flatness issue.
I've not experienced health problems with the fumes from RA4 chemicals but for what it's worth I once replaced an air conditioner in the darkroom and found that the copper tubes within were oxidized black and partially corroded, presumably from the chemicals in the atmosphere which would have been predominately RA4. OzJohn