Second this! When I first started writing for fandom, someone offered to beta for me and as I had never done it before, I just gave them access to my google doc and then was crushed when they rewrote full paragraphs so that they didn't sound like my writing anymore. I think I took some of them out, and then they were pissed at me for that. Not specifically their fault, since I was new to it and didn't yet know what I wanted in a beta and so we didn't set any expectations beforehand, but I very quickly realised that that wasn't it.
Ever since then I have only ever turned suggestion mode on, and communicated clearly with later betas on what I want. For me, that is mostly SPAG (for some fics, it's even just SPAG), because I mostly write short fics where I (think I) can keep plot consistent in my head, and then for (mostly longer) fics where I want more feedback, I tell my betas how I want it given so that we can best work together. In my case, that's usually telling me in which places things are unclear, either from phrasing or what is happening, and if I should rephrase something, I have them tell me which sentence/paragraph and what doesn't work, but it gives me the freedom to use my own words for the rewrite, so the work still feels like my own. Then I run the changed sentence/paragraph by them to see if it's clearer and then put it in. Or if I can't think of ways to rephrase, then I ask for suggestions on how to rephrase it.
This also puts you in a conversation, it's not "the beta says this has to be x", but for me it's usually been like "you can put it like x" and I'll be like "hm that is not quite what I was looking for, it sounds like it has y connotation and I was looking it to sound more like z" and then them "what about this instead?". If you're looking for connection and conversation in fandom, I find this is the way to go, because then a beta-writer relationship, no matter if just for one fic or for longer, doesn't feel transactional and instead feels like working together on a project and talking about something (your fandom, your ship, your au, your fic) that you care about.
As far as I remember, I've only ever had positive experiences with this approach because it means no one is disappointed. The writer isn't disappointed because the text no longer sounds like them or the beta replaced full passages, and the beta isn't disappointed because the writer changed their edits to something else.
As the above poster said, when I first started writing for fandom, I didn't know there was different ways to beta and that I was allowed to say "I don't care about consistency in this specific work, I just want the grammar to be fixed" or "I'd only like you to suggest and then reword things myself". The first few times I had betas after that first experience, I felt like I had to justify why I wanted a specific type of feedback and it took some comments from friendly betas encouraging me to voice how I'd like feedback to be given and them telling me that they like it that I have clear instructions for what I want in beta reading.
But you are allowed. Sure, sometimes a beta will have to be like "I don't think I'm comfortable with this type of beta reading", but that is okay because otherwise you'd have both been unsatisfied with the experience. Better to know it upfront than to find out at the end that one or both of you hated the experience or the end product.
Tell your prospective beta what you expect from beta reading, let them tell you what they expect from you, find out if that is compatible or not, and then have fun writing and beta reading and talking about the work!
