I was attending a conference in Budapest and tacked on a few days there and in Poland. I'll especially try to note where my experience deviated from the 2019 Eastern Europe guide but am happy to answer any other questions.
FLIGHTS
There are no flights direct to Budapest from Philly or Newark, so I got a ticket on Lufthansa, Philly to Frankfurt to Budapest. Sounds like I'm not the only person disappointed this summer by buying a Lufthansa ticket and finding myself boarding a "Eurowings Discover" flight. Overall I found that airline a bit chintzy, I can't exactly point to anything wrong with it, except that we did leave an hour late, and somehow did not make up any of the time in the air. Lots of connection drama in Frankfurt, I only made mine since that flight was also very late.
MONEY
I didn't use much cash--I actually paid for most things with Apple Pay on my watch, connected to a credit card. In addition to ATMs trying to scare you into choosing dollars and getting their rotten exchange rate, I also found this with credit card terminals (more in Poland than Hungary). Since I know my banks give me a good rate with no international fees, I always chose to process transactions in forint/zloty, but keep an eye out for this. It's a real "dark pattern" design.
BUDAPEST
I had been to Budapest before but it was in the 90s. Obviously with so much time having passed, things were extremely different. No Trabbis! I had 1.5 days of sightseeing before the conference started.
I stayed at the Hotel Corvin, selected by the conference organizers. it had 4 and 3 star sections and was an easy tram ride away from the central area. It was scrupulously clean, the breakfast was good, and the staff were helpful. I think RS travelers would like it.
Prices for sights are generally much higher than the 2019 guide says--say 180% for a number of the places I visited. For example the Parliament and Great Synagogue are around $30 at current exchange rates and take only an hour or so to see. So much for Europe on $5 a day!
I had a transit pass during the conference but beforehand I was trying to pay per ride. Instead of paying cash to the driver, there is now an app, Budapest Go, which you can theoretically use for this purpose. I found it to be poorly implemented and the staff were unable to give consistent answers to easy questions like "can you switch from one metro line to another on the same e-ticket or do you have to pay again." They were checking tickets/passes in tourist areas in an aggressive, I'd even say rude, way. My advice in retrospect would be to just buy a pass for whatever period of time you'll be there and not worry about any of this. Uber still does not exist in Hungary and at least once I decided not to take a cab because the price the driver quoted was so outlandish.
Some sights in short: Parliament--tour was about 45 minutes, I think I'd been there on my previous trip, but worth it because it's so unique. Market Hall--interesting to walk through, wouldn't consider it a must unless it's on your way. Great Synagogue--a highlight, and the English tour guide was very good (tour was free with the expensive admission). I had lunch afterward at Mazel Tov and enjoyed it. Mattias church and wandering around on the Buda side--RS guide seemed right on the money for what and how much to see walking around this area.