I’m with Swissie on this, although I tend to space my boosters as evenly as possible every 6 months. I rely a lot on the Singapore Ministry of Health (MOH) for getting up to date and balanced advice on this matter, and basically what they are saying, for the time being, to be considered up to date with your COVID 19 vaccinations, you should get a booster 5 - 12 months after your last shot (refer to the SG MOH web page: https://www.moh.gov.sg/covid-19/vaccination ).
I don’t think you can be “over-vaxxed” to the point of too much vaccine causing health problems, but at this time, take the balanced and practical view, read as much expert advice as you can and then come up with your own program that fits in with this advice. I had 3 Pfizer shots in Singapore in 2021 and since arriving in Thailand, I’ve had 2 Moderna boosters, both of which I paid for. I note that most of my friends and colleagues are quite relaxed about their vaccination status, and don’t see the need to keep up to date - that’s their choice, but I prefer to be more proactive in managing my health..