It was nice to meet you at Field Day Scott, congratulations on getting your first HF rig.
Well done on two great choices of kit to get you going on HF.
I have a Yaesu FT-891 although I've only used it twice as I got it just before heading back out here to Canada, preferring to bring my trusty 857 with me as I wanted all mode VHF capabilities while here.
The LDG Z-100 (now discontinued) I showed you at field day is the forerunner to the Z-100 plus, but I think they are the same internally and functionally. Both my 857 and Z-100 have performed flawlessly for around 13 years now.
I assume you are using the optional interface cable between the tuner and the rig? If not I would recommend it as it makes life much easier, all you need to do is press tune on the ATU and wait a few seconds for it to work its magic and away you go.
Here is the link for the interface cable if you need it..
Well done on your first contacts, hopefully as you improve your set up there will be plenty more DX to come. Recently I've worked some pretty good DX while out portable, quite a few stations in Europe worked as well as Caribbean and South America. Conditions on 20 on a Saturday were pretty good during the ITU contest, in an hour of operating I got 11 new countries.
You mention moving to an upstairs shack, but also working on a dipole. Others may have had more luck but speaking from my experience achieving a decent/practical (low impedance) RF earth from an upstairs shack can be tricky if not impossible. If you are going to be using a balanced antenna such as a dipole you would likely be better off without one as having a lengthy earth conductor may actually cause you RFI problems. If using a dipole make sure you use a 1:1 balun at the feed point for the same reasons.
An artificial earth such as an MFJ 931 or 934 may help in an upstairs shack if using an end fed or other unbalanced antenna. Another option is to use a fully automatic remote ATU with a decent RF ground at the feed point but for now I would see how you go with your plans without spending more than you need.
If you do have RF getting back into you shack you will soon know, aside from it maybe causing issues with other electronic devices in your home, or your neighbours you would likely soon get reports of distorted audio, a sure sign.
Trying different antennas, improving you station and resolving problems as they arise is all part of the challenge and fun of the hobby. Having a great club such as GBARC where you can turn to others to share their experiences face to face or through this forum is priceless.
Do let us all know via the forum how you are getting on, good luck and 73..