Convenient hotels near the conference venue:
Hotel COOP Sofia , https://www.coophotel.bg/en/
Eurostars Sofia City , https://www.eurostarshotels.co.uk/eurostars-sofia-city.html
Crystal Palace Boutique Hotel , https://www.crystalpalace-sofia.com/
InterContinental Sofia by IHG , https://www.ihg.com/intercontinental/hotels/gb/en/sofia/sofco/hoteldetail
All these hotels are situated about (or less than) 10 minutes walking distance from the conference venue.