In order to avoid buying high, you need to know what "high" means. Is Ethereum high right now? In a sense yes, because otherwise it wouldn't be hovering around its all time high, but then it is also possible that it breaks out and goes towards four figures at full speed. Really tough to tell here and I wouldn't bet my house on neither of both possibilities. Today it looks very healthy and stable, but that could change tomorrow. Just don't go all in, buy in tranches and build up your position over time no matter which direction it takes in the short-term.
The market is so volatile and the possible price that Ethereum may attain in any moment is unpredictable so if you are ready to take the risk then why not there is no such perfect timing to buy ETH. I would like to share my regretful moment on my trading this past weekends, I was at the market and looking for a good to buy token for day trading, I check BTC, ETH, SOL and other token that I know worth on a day trade, at that time ETH was dipping around $3.6k I hesitate and I didn't bought it, just an hour later I check ETH and the price starts pumping, the pump that I know it will keep rising and I ended up missing to get a profit because of my hesitation. Now the market of ETH is very healthy in fact it attained its new ATH so it's up to you if you would like to take a chance to invest or wait.