With Thunderball, Moonraker, Die Another Day and Spectre, I'm sure most of us agree that these are inferior to each actors' respective 3rd films and there is an apparent pattern for a fall-off in quality on the 4th entry.
The first two entries try to be for the Bond fans. They try to establish the story and setting, set a new tone for the series, draw people in on the acclaim. Then, the 3rd film usually perfects the formula and is that breakout film (Goldfinger, Skyfall, TSWLM). At that point, the franchise is at the top in acclaim, it has a bigger budget and it wants to reach new markets. The creative minds at EON convince the company that Bond is immensely profitable. So EON spends a ton of money back on slick action scenes to reach an audience of filmgoers that like expensive action (but don't really care about the spy aspect). All 4 of the 4th films have inflated budgets and at least 3 of them are shortchanged on the plot. Thunderball is the exception, but even that film falls for the "bigger is better" mentality.
Another reason for this focus on grandiosity as the top priority is perhaps because EON is afraid that Bond films will become outdated and be the establishment, so they need to get with the times and adopt a lot of tech-y and trendy elements to appeal to young people. This happens a lot in music when the artist gains a lot of new fans, has a reputation, wants to break their prior sales numbers to become a total giant, but also they no longer feel they have to prove merit they've proved already.