If clauses are as independent as that then there is no grammatical relationship between their tenses. There may be potential semantic issues, but that would be very much dependent on the specific details. In your example it is absolutely fine. You could separate them with a full stop rather than a comma, switching but for however, and some people would say that is stylistically better. Many style guides are fine with it as you have it, with or without a comma. As long as there's either a comma or a linking word (or, often preferably, both), it's not a run-on sentence, which would be ungrammatical. Here it's not ungrammatical, as you have a conjunction and a comma.
Tense consistency in English is not something you can make clear rules for. There is no overall grammatical rule requiring consistency between different verbs in the same sentence, especially where they are in separate clauses or phrases. In the first clause of your sentence you have both present and past - you are sorry (now) that you could not (in the past) see them before leaving. In the second clause you will visit (in the future) and you hope (now) to see them then. This is normal, and nothing to worry about.
The tense should be the same when describing the same action or activity. You can run into such problems if you start in one tense but finished in another (spot the deliberate error). However, that is one action in one conditional being described by two verbs. One sentence or clause can contain individual phrases that relate to very different actions. The thing is to be aware of semantic contradictions. Tense combinations are effectively driven by meaning, not grammar.