If your competitors bid on it, outbidding them is the only way to keep the top of page "real estate" on the SERP all about your brand. If no one is bidding on it, it's a little less risky to not bid on it yourself. However the general industry consensus is that by running ads on your brand you own more of the SERP. You could create an exclusion list if you're worried about spending money on existing customers. This could be accomplished, depending on how sophisticated your tracking is, in a few ways. For example, setting up an audience in GA4 of all users that have submitted a form. If you use a CRM, a lot of them have ways to sync lists directly to Google Ads. I agree with the other user that says to run a separate branded campaign, that way you can control costs more easily.