Linguistics

I see two ways to do so:

1. Have multiple agent noun affixes, each for a type of agent. For example I feel like Spanish -dor is more often used for someone who's repeatedly doing something, while -nte is someone doing it now.
2. Apply the affix not to the base form of the verb, but to a conjugated form, in a way that preserves tense/aspect/mood information.

So, as an example of #2. Let's say your conlang has the verb "lug" (to do), and here's part of its conjugation:

• indicative perfect past - lugene (they did)
• indicative imperfect past - lugavo (they were doing)
• indicative habitual present - lugien (they often do, they typically do)
• indicative progressive present - lug (they're currently doing)
• [etc.]

And your agent suffix is, dunno, -bor. Most languages would apply it into the base form and call it a day, so you'd get "lugbor"; you could instead do something like

• lugenebor - the one who did
• lugavobor - the one who used to do
• lugienbor - the one who often does; like Spanish "hablador" (one who talks often = talkative)
• lugbor - the one actively doing it; like Spanish "hablante" (one who's talking now = speaker)
• etc.

I feel this would go well with an agglutinative language. Just make sure the distinction between adjective and noun is clear, otherwise your conspeakers will conflate the nominalising and adjectivising suffixes.