The English verb “suffer” has several meanings. In 2 Peter 3:9, it is used in the now obsolete sense of:1
intransitive. To endure, hold out, wait patiently. (Often with abide, bide.) to suffer long: to be long-suffering. Obsolete.
In modern English, we could translate it as “is patient.” Yes, both “to be longsuffering” (now obsolete) and “to be patient” are appropriate translations of the Greek verb μακροθυμέω.
The Oxford English Dictionary also has the following entries on the noun long-suffering and the adjective long-suffering.
Origin: Formed within English, by compounding; modelled on a Greek lexical item. Etymons: long adj.1, suffering n.
Etymology: < long adj.1 + suffering n., ultimately rendering Hellenistic Greek μακροθυμία (see longanimity n.).
Chiefly in religious contexts.
Patient endurance of provocation or trial; forbearance, longanimity.
Origin: Formed within English, by compounding. Etymons: long adv.1, suffering adj.
Etymology: < long adv.1 + suffering adj., after long-sufferance n., long-suffering n.
In quot. 1535 perhaps influenced by German langmütig (see longmood adj. at long adj.1 and n.1 Compounds 4a), itself after classical Latin longanimus and ultimately after Hellenistic Greek μακρόθυμος (see longanimous adj.). The Hebrew original has 'ereḵ appāyim slow to anger.
Patient or forbearing in spite of troubles or provocation.
1 Oxford English Dictionary, “suffer,” v., 6.
“long-suffering, adj.” OED Online, Oxford University Press, June 2020, oed.com/view/Entry/110083. Accessed 8 August 2020.
“long-suffering, n.” OED Online, Oxford University Press, June 2020, oed.com/view/Entry/110082. Accessed 8 August 2020.
“suffer, v.” OED Online, Oxford University Press, June 2020, oed.com/view/Entry/193523. Accessed 8 August 2020.