Future perfect tense sentences in Chinese (eg. "Next month I will have been married 3 years"
You'll know when you get to Beijing 你到了北京就会知道的
You'll know when you get to Beijing 你到北京就会知道了

的 is not the right choice of final particle here.
Edit:
就会知道 = will then know
in 就会知道(的), '的' is a final particle that emphasizes the verb, stating it is in an existing state', that contradicts 就会(will then).  You can say &quot;他是知道的&quot; (he knows/ he knew) but not &quot;他將是知道的 (he will knows/knew)
in 就会知道(了), '了' is a final particle that indicates completion of the verb. Meaning 知道了 indicates the verb 知道 will be 'completed'. Since '了' is an 'aspect marker' not a 'tense marker', it can be used in any tense.

Next month I'll have been married for 3 years 等到下个月我就结婚三整年了
Next month I'll have been married for 3 years 下个月我就结婚整整三年了

It is [整整 X year (X years no more no less)], not [整 X year]
Your English sentence didn't state 'wait until' so you shouldn't write '等到' In your Chinese translation

Next year I'll have been to Beijing (and left) 等到明年就去过北京的
Next year I'll have been to Beijing (and left) 明年我就会去过北京一趟了

From the context, we know you've never been in Beijing, therefore &quot;去过北京(一趟)&quot; can make it more clear that you mean 'go and leave Beijing (been in Beijing)'
But it is English-Chinese, Native Chinese would phrase it as &quot;明年我会去北京一趟&quot;  - &quot;I will take a trip to Beijing next year (and leave)&quot;

After I've gone to Beijing, I'll plan where to go next 我去了北京以后就打算后面去哪里
After I've gone to Beijing, I'll plan where to go next 去过北京之后我才再想下次要去哪里

You said: &quot; After I've gone to Beijing&quot; meaning &quot; After you had been in Beijing and back&quot; (去过北京之后)

(then) I'll plan where to go next

才 = &quot;only then&quot; (you won't make plan for the next destination until you finished visiting Beijing)
打算 means &quot;intend to&quot; But you already intended to go somewhere else after the Beijing trip, so you should either use 计划 (to plan) or simply use 想 (think about/plan)

I'll have finished by that time 那时候我已经做完了 (is the 了 right here even though I'm talking about the future?)
I'll have finished (my traveling plan) by that time 那时候我就完成了我的旅行计划了

'完成了' (completed) already indicate the action is completed, no need to add '已经' (have been already)
了 is an aspect marker that indicates 'completion of an action' it can be used in any tense.