It is not convient to go to a Lexus dealership for interval maintenenace. Can a Toyota dealership do it and the Lexus warranty still be intact?

Anybody, including you, can perform maintenance without affecting the warranty. Just document the maintenance and consider keeping relevant receipts. At least occasionally check to see if any TSIB's or other service campaigns have been issued for your car -- a lot of free work gets done during the warranty period without asking. You can see the TSIB's on the NHTSA, ARRC, and other websites. Maybe it's changed, but in the early Lexus years, TSIB's were not always issued for known problems -- I never did understand these "secret" service campaigns.