To apply to PhD eventually, how important is it to do an MS program with thesis if I have strong undergrad research experience?
Your concern about professor focus in a research-focused MS seems out of place when you have what seems to be a lot of research experience as an undergrad - and I don't know of anywhere where undergrads get more help/focus related to research than at a research-focused grad school, even at the MS level. Yes, it can be a concern in making sure you pick a good program and a good adviser - but that's always true no matter what you do in academia!

If you just want an MS to prove you can get a good GPA, honestly that seems like an uninspiring goal to have for 2 years of work. I would be concerned for your enjoyment of the time and ability to stay motivated if research and a PhD is your interest. And being out of the research loop will not help you write a good application proposal for research, or help you identify more specifically what research you want to do and who you want to work with - which are perhaps the most potentially valuable things to get from an MS if you want to continue with research.

My other concern for you is that all that research as an undergrad looks great - while you are an undergrad. But people almost always weight recent information more heavily than older info. So if you apply to grad schools and write about all the research experience in undergrad, and how well you did in courses in an MS...that's exactly backwards, and it seems like that would make someone think you weren't really that interested in research. I mean, you started in undergrad - if you really enjoyed it realized you wanted a PhD then, how did you take 2 years off research to focus on grades?

I'm not saying your proposed plan would be a kiss of death or anything, but it seems like a non-optimal strategy. I'd suggest you think now how you will talk about the experience in a PhD application, if that is your goal, and what story will be the most compelling and intelligible. And what will truly prepare you for the best to have a great PhD experience, and beyond? Keep in mind that after you are admitted to the program, grades only matter in that you need to not fail classes - but beyond that, no one seems to care - not for finishing the PhD, and not for your career beyond.