Arriving in Bath a day early for the Rick's Best of England tour.

Call Rick Steves tour office and they’ll tell you. I would imagine Bath Abbey & the Roman Baths would be included but verify with them.

One of the things I really enjoyed in Bath was spending a few hours at.the Thermae Bath Spa. I love experiential travel & wanted to experience the hot springs and it was a treat that I absolutely loved & will always remember.

You are right...the tour covers the Royal Crescent and the Roman Baths.

The interior of Bath Abbey was not covered on our Rick Steves Best of England tour. The exterior was included on a walking tour given by our tour guide.
This walk also included Queen's Square, with an explanation of its townhouses.

The Holburne Art Museum is not included on the tour, so you might be interested in going there.
https://www.holburne.org/

This is from your itinerary for your tour:

Our day begins with a walking tour of Bath's charming streets, including its medieval abbey square and the imposing sweep of its Royal Crescent. Next we're off to tour the town's remarkably preserved, 2,000-year-old Roman Baths, where you'll have a chance to drink from these natural mineral springs in the Pump Room. The rest of the day is free to make your own discoveries in this historic town. You might visit the colorful Fashion Museum or — as Roman travelers did — bathe in a mineral spa.

Another interesting thing to do might be the bus tours of the city. These leave from in front of the Tourist Information Office, where you can buy tickets. Or you can just get on and pay the bus driver.
There are two bus routes, one around town; the other takes you out of town, by the train station, up into the hills, past the American Museum (there's a stop here in case you want to hop off), then back down into town.
Most interesting to do. Both rides are included in the ticket. Sit upstairs in the buses for a better view.
These bus rides have a narrator/guide telling you about the sights you are seeing. (It's not expensive.)

https://visitbath.co.uk/plan-your-visit/visitor-information
https://visitbath.co.uk/plan-your-visit/maps-of-bath
https://visitbath.co.uk/things-to-do

We will be going on the Best of England tour starting Sun 8/13, arriving 3 nights early so we can do a couple day trips from Bath (a Cotswold day trip, and a Salisbury/Stonehenge day trip), but I am also planning to see and do the following extra sights in Bath that aren’t necessarily covered by the tour:

• The mayor of bath corps of honorary guides FREE walking tours (free, 2hrs, tues/thurs 6pm, Sun-Fri 10:30am/2pm, Sat 10:30am only). Start/end at pump room
• No 1 Royal Crescent museum (£15/£13.50, 10am-5:30pm, closed mon)
• walk along Bath canal towpath (90min, 2 mile loop). Audiostops
• Parade gardens (£2/£1, 10am-6pm)
• Bath Abbey (4£ donation, 10am-5:30pm, sat -6pm, sun 1:15pm-2:30pm & 4:30pm-6:30). Best of Abbey tour (£8, inquire times). Tower tour (£10, top of hour? 50 min)

I wanted to visit the fashion museum also, but it is unfortunately closed this year as they are moving locations.

In your account page, under Pre-Tour Planning, it says:
Sightseeing on Your Own
If you arrive in Bath before your tour begins, use the Rick Steves Great Britain guidebook to get oriented and find information about visiting the sights listed below.

Here are a few suggestions for sightseeing in Bath that we will not do as a group: Evensong at Bath Abbey, Georgian House at No. 1 Royal Crescent, Fashion Museum, Assembly Rooms, Museum of Bath at Work, City Bus Tours, day trip to Stonehenge or Salisbury; or attend the theater in London.

As noted above, you should check on any of them before you plan on them, as some (like the Fashion Museum) may not be available.