1. TaxPhD (another frequent visitor here) can probably answer a little better than me. Here are a few schools off the top of my head. UT Austin and UNC have been strong in the field for a long time. MIT and UPenn are excellent programs with some senior tax people. UC Irvine Has Terry Shevlin, but I would be interested in his retirement plans before applying there. Oregon has senior people. I think Iowa still does, they had a tax grad last year place well. UIUC is fairly strong in all areas. I think Texas A&M has graduated tax people recently. Arizona had a strong history of tax, but their most senior faculty member passed away.
Additionally, there are lots of schools that provide solid training in the exact same research methods, but only have more junior tax people without a long history of training tax students. If you branch out, you should look at schools like this. OSU, UGA, Duke, Washington, Stanford, Wisconsin, Michigan, Michigan State, Florida, ASU. This is definitely not a complete list and I am probably missing some senior faculty at at least one of these schools.
The BYU Ranking for tax is a great place to start investigating stuff so you can look into programs yourself: Tax_ArchiAccounting Research Rank
2. It doesn't really matter for you. You already have the experience, so this isn't affecting any decisions. That said, I think there is still a positive effect of more work experience, but there is decreasing marginal benefit after about 2-3 years. The people that I have known with 8+ years of experience going into a program have all done fairly well. They all struggled with coursework, but their experience clearly informed research interests and they were able to talk about details of institutional settings very easily. Some experience is quite common in tax, so I wouldn't worry about your age or experience being negative parts of your application.