I agree with the advice to standardize your bar size as much as possible. A slab mold in conjunction with the molds you have could work if you can cut your loafs from the slab to get the size you want without too much waste.
Awhile back I was trying to sell a large slab mold from Soap Equipment for a friend of mine. It is a nice mold, however I will caution you that the HDPE is not as smooth as silicone. I am admittedly bothered by this kind of thing, and personally would plane all the texture off, even though it is minimal. I can check with her to see if she still has it. I know she was planning to see if she could sell it on FB Marketplace or find someone who wanted it. It was
this mold.
I'm pretty happy with my WH triple T&S mold, although I did have a liner rip. WH admitted to having some issues with the silicone supply around the time I purchased my mold and replaced it. Unfortunately, the silicone replacement was a bit thicker which made my dividers not fit. I was able to grind them down to make them work, but the fact that they didn't care left me not likely to purchase from them or recommend them again. YMMV, of course, and the mold is very nice.
I have the large NS slab. Hubby bought it as a gift when I first started making soap. I hardly ever use it because it is so big, and since I don't sell it just makes too much soap. I really only used it a few times for large donation batches to a local veteran's shelter. PM me if you are interested - I might be persuaded to part with it! Someone might as well put it to use.
Also, if you decide to go with a new slab mold and need a cutter for that I would hands down invest in the Caterpillar from Custom Craft Tools. The products this company makes are among the best soaping tools out there. It's expensive though and no, I don't have one