I have a 1975 built house where I want to remodel the lower floor bathrooms. At this point, all drain and vents are original to the home. The main stack rises out of the ground and then is a straight 3 copper vertical line right up through the roof. All bathroom plumbing on both floors is wet vented to this stack. In the second floor the toilet drains into a closet bend with a short 5 or so extension and then tees to the stack. That extensions contains a 1 tee that runs about 36 to the bathtub p- trap. About 18 above the main stack toilet tee another 1/1/2 tee serves a horizontal 36 line to the sink trap. On the first floor, One toilet connects in a similar fashion to the main stack with a 8 extension to the bend and also with a 1/1/2 bathtub drain teed into the extension before the connection to the stack. There is a second toilet served by a 3 pipe running about 48 teed directly in to the stack just below the other toilet . In addition to the above, which I know for certain, I have a sink that I believe simply drains horizontally 72 to the stack, probably 1 pipe. There is no vent attaching to the stack above the 2nd floor fixtures so I think it is simply wet vented also.

I understand the concept of wet venting but wonder about the critical distances and other requirements that I am unsure of. That said, this is a vacation house and in 30+ years of very heavy weekend use often by a lot of people(including a washing machine also teed directly in that stack) there has never been the slightest issue with the plumbing.

Now we want to remodel due to an accessibility need. The first floor tub is being changed to a shower. So I understand I need to upgrade to a 2 drain for the shower. Also a sink is being moved to where it needs to run an extra 3 feet horizontally or it could be run down through the floor and then horizontally to the stack, probably shorter than the original 72.

Can anyone comment on the best arrangement for that sink in terms of drain and vent requirement?
Would anyone comment of the current entirely wet vented configuration. I was surprised when I found no other vents on any of the fixtures.